Exhibit 1.1

6,000,000 Shares

CERUS CORPORATION

COMMON STOCK, PAR VALUE $.001 PER SHARE

UNDERWRITING AGREEMENT

June 5, 2003

Morgan Stanley & Co. Incorporated

1585 Broadway

New York, New York 10036

Dear Sirs and Mesdames:

Cerus Corporation, a Delaware corporation (the “Company”), proposes to issue and sell to Morgan Stanley & Co. Incorporated (the “Underwriter”) 6,000,000 shares of its common stock, par value $.001 per share (the “Firm Shares”). The Company also proposes to issue and sell to the Underwriter not more than an additional 900,000 shares of its common stock, par value $.001 per share (the “Additional Shares”) if and to the extent that the Underwriter shall have determined to exercise the right to purchase such shares of common stock granted to the Underwriter in Section 2 hereof. The Firm Shares and the Additional Shares are hereinafter collectively referred to as the “Shares.” The shares of common stock, par value $.001 per share, of the Company to be outstanding after giving effect to the sales contemplated hereby are hereinafter referred to as the “Common Stock.” Each share of Common Stock has rights (the “Rights”) attached thereto to purchase Series C junior participating preferred stock (“Series C Preferred Stock”) issued pursuant to a Stockholder Rights Plan dated November 3, 1999 (the “Rights Agreement”),

The Company has filed with the Securities and Exchange Commission (the “Commission”) a registration statement, including a prospectus (the “Base Prospectus”), relating to the Shares, and has filed with, or transmitted for filing to, or shall promptly hereafter file with or transmit for filing to, the Commission a final prospectus supplement (the “Prospectus Supplement”) specifically relating to the Shares pursuant to Rule 424 under the Securities Act of 1933, as amended (the “Securities Act”). The term “Registration Statement” means such registration statement, including the exhibits thereto, as amended to the date of this Agreement. If the Company has filed an abbreviated registration statement to register additional shares of Common Stock pursuant to Rule 462(b) under the Securities Act (the “Rule 462 Registration Statement”), then any reference herein to the term “Registration Statement” shall be deemed to include such Rule 462 Registration Statement.

The term “Prospectus” means the Base Prospectus together with the Prospectus Supplement. All references to the Registration Statement, Base Prospectus, or Prospectus shall include documents incorporated therein by reference. The terms “supplement” and “amendment” or “amend” as used in this Agreement with respect to the Registration Statement or the Prospectus shall include all documents subsequently filed by the Company with the Commission

pursuant to the Securities Exchange Act of 1934, as amended (the “Exchange Act”), that are deemed to be incorporated by reference in the Prospectus.

1. Representations and Warranties. Except as described in the Registration Statement or Prospectus, the Company represents and warrants to and agrees with the Underwriter that:

(a) The Registration Statement has become effective; no stop order suspending the effectiveness of the Registration Statement is in effect, and no proceedings for such purpose are pending before or threatened by the Commission.

(b) (i) Each document, if any, filed or to be filed pursuant to the Exchange Act and incorporated by reference in the Prospectus complied or will comply when so filed in all material respects with the Exchange Act and the applicable rules and regulations of the Commission thereunder, (ii) the Registration Statement, when it became effective, did not contain and, as amended or supplemented, if applicable, will not contain any untrue statement of a material fact or omit to state a material fact required to be stated therein or necessary to make the statements therein not misleading, (iii) the Registration Statement and the Prospectus comply and, as amended or supplemented, if applicable, will comply in all material respects with the Securities Act and the applicable rules and regulations of the Commission thereunder and (iv) the Prospectus does not contain and, as amended or supplemented, if applicable, will not contain any untrue statement of a material fact or omit to state a material fact necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading, except that the representations and warranties set forth in this paragraph do not apply to statements or omissions in the Registration Statement or the Prospectus based upon information relating to the Underwriter furnished to the Company in writing by the Underwriter expressly for use therein.

(c) The Company has been duly incorporated, is validly existing as a corporation in good standing under the laws of the jurisdiction of its incorporation, has the corporate power and authority to own its property and to conduct its business as described in the Prospectus and is duly qualified to transact business and is in good standing in each jurisdiction in which the conduct of its business or its ownership or leasing of property requires such qualification, except to the extent that the failure to be so qualified or be in good standing would not have a material adverse effect on the Company.

(d) The Company does not own or control, directly or indirectly, any interest in any other corporation, association or other business entity.

(e) This Agreement has been duly authorized, executed and delivered by the Company.

(f) The authorized capital stock of the Company conforms as to legal matters to the description thereof contained in the Prospectus.

(g) The shares of Common Stock outstanding prior to the issuance of the Shares have been duly authorized and are validly issued, fully paid and non-assessable, and the Rights attached to such shares of Common Stock have been duly authorized, and the Series C Preferred Stock related to such Rights has been duly authorized and validly reserved for issuance upon the exercise of the Rights in accordance with the terms of the Rights Agreement.

(h) The Shares have been duly authorized and, when issued and delivered in accordance with the terms of this Agreement, will be validly issued, fully paid and non-assessable, and the issuance of such Shares will not be subject to any preemptive or similar rights and the Rights attached to those Shares have been duly authorized, and the Series C Preferred Stock related to such Rights has been duly authorized and validly reserved for issuance upon the exercise of the Rights in accordance with the terms of the Rights Agreement.

(i) The execution and delivery by the Company of, and the performance by the Company of its obligations under, this Agreement will not contravene any provision of applicable law or the certificate of incorporation or by-laws of the Company or any agreement or other instrument binding upon the Company that is material to the Company, or any judgment, order or decree of any governmental body, agency or court having jurisdiction over the Company, and no consent, approval, authorization or order of, or qualification with, any governmental body or agency is required for the performance by the Company of its obligations under this Agreement, except such as may be required by the securities or Blue Sky laws of the various states or the Commission or the Nasdaq National Market in connection with the offer and sale of the Shares.

(j) There has not occurred any material adverse change, or any development involving a prospective material adverse change, in the condition, financial or otherwise, or in the earnings, business or operations of the Company, from that set forth in the Prospectus (exclusive of any amendments or supplements thereto subsequent to the date of this Agreement).

(k) There are no legal or governmental proceedings pending or threatened to which the Company is a party or to which any of the properties of the Company is subject that are required to be described in the Registration Statement or the Prospectus and are not so described or

any statutes, regulations, contracts or other documents that are required to be described in the Registration Statement or the Prospectus or to be filed as exhibits to the Registration Statement that are not so described or filed as required.

(l) The Company is not, and after giving effect to the offering and sale of the Shares and the application of the proceeds thereof as described in the Prospectus will not be, required to register as an “investment company” as such term is defined in the Investment Company Act of 1940, as amended.

(m) To the best of the Company’s knowledge, the Company (i) is in compliance with any and all applicable foreign, federal, state and local laws and regulations relating to the protection of human health and safety, the environment or hazardous or toxic substances or wastes, pollutants or contaminants (“Environmental Laws”), (ii) has received all permits, licenses or other approvals required of it under applicable Environmental Laws to conduct its business and (iii) is in compliance with all terms and conditions of any such permit, license or approval, except where such noncompliance with Environmental Laws, failure to receive required permits, licenses or other approvals or failure to comply with the terms and conditions of such permits, licenses or approvals would not, singly or in the aggregate, have a material adverse effect on the Company.

(n) To the best of the Company’s knowledge, there are no costs or liabilities associated with Environmental Laws (including, without limitation, any capital or operating expenditures required for clean-up, closure of properties or compliance with Environmental Laws or any permit, license or approval, any related constraints on operating activities and any potential liabilities to third parties) which would, singly or in the aggregate, have a material adverse effect on the Company.

(o) There are no contracts, agreements or understandings between the Company and any person granting such person the right to require the Company to file a registration statement under the Securities Act with respect to any securities of the Company or to require the Company to include such securities with the Shares registered pursuant to the Registration Statement, except as have been or prior to the Closing will be appropriately waived pursuant to the terms and provisions of such contracts, agreements and understandings.

(p) The Company has all consents, approvals, orders, certificates, authorizations and permits issued by, and has made all declarations and filings with, all appropriate federal, state or foreign governmental or self-regulatory authorities and all courts and other tribunals necessary to conduct its business and to own, lease or license and use its properties in the manner described in the Prospectus, except for

such consents, approvals, orders, certificates, authorizations, permits, declarations and filings the failure of which to have, maintain or make would not have a material adverse effect on the Company, and the Company has not received any notice of proceedings relating to the revocation or modification of any such consent, approval, order, certificate, authorization or permit.

(q) Except as disclosed in the Registration Statement or Prospectus, the Company owns or possesses, or has rights to all material patents, patent rights, licenses, inventions, copyrights, know-how (including trade secrets and other unpatented and/or unpatentable proprietary or confidential information, systems or procedures), trademarks, service marks and trade names currently employed by it in connection with the business now operated by it or proposed to be operated by it as described in the Registration Statement or Prospectus, and the Company has not received any notice of infringement of or conflict with asserted rights of others with respect to any of the foregoing which, singly or in the aggregate, if the subject of an unfavorable decision, ruling or finding, would have a material adverse effect on the Company.

(r) There are no contracts or other documents, relating to patents or patent applications owned or licensed by, or licenses of patents or patent applications held by, the Company of a character required to be filed as an exhibit to the Registration Statement, to the Company’s annual report on Form 10-K for fiscal 2002 or to the Company’s quarterly reports on Form 10-Q for the first fiscal quarters of 2003, or required to be described in the Registration Statement or Prospectus or in such Form 10-K or Forms 10-Q that are not so filed or described.

(t) The clinical trials and the human and animal studies conducted by or on behalf of the Company or in which the Company has participated that are described in the Prospectus were and, if still pending, are being conducted in accordance with standard medical and scientific research procedures and any applicable rules, regulations and policies of the U.S. Food and Drug Administration (the “FDA”).

(u) The Company has operated and currently is in compliance in all material respects with all applicable rules, regulations and policies of the FDA and comparable regulatory agencies in Canada and in each European country in which the Company’s products are marketed (collectively, the “Regulators”).

(v) Except as described in the Registration Statement and the Prospectus, the Company is not required to file or obtain any registration, application, license, request for exemption, permit or other regulatory authorization with the Regulators in order to conduct its business as described in the Registration Statement and Prospectus.

(w) Except as described in the Registration Statement and the Prospectus, to the knowledge of the Company, the Company holds, and is operating in compliance in all material respects with, all franchises, grants, authorizations, licenses, permits, consents, certificates and orders of the Regulators required to conduct its business as described in the Registration Statement and Prospectus, and all such franchises, grants, authorizations, licenses, permits, consents, certifications and orders are valid and in full force and effect; and, as of the date of this Agreement and except as described in the Registration Statement and the Prospectus, to the knowledge of the Company, the Company is in compliance in all material respects with all applicable federal, state, local and foreign laws, regulations, orders and decrees.

(x) Neither the Company nor, to the knowledge of the Company, its distribution partners, have received notification of the violation of any applicable statute, rule, regulation or order administered or issued by any of the Regulators.

2. Agreements to Sell and Purchase. The Company hereby agrees to sell to the Underwriter, and the Underwriter, upon the basis of the representations and warranties herein contained, but subject to the conditions hereinafter stated, agrees, to purchase from the Company the Firm Shares at $9.05 a share (the “Purchase Price”).

On the basis of the representations and warranties contained in this Agreement, and subject to its terms and conditions, the Company agrees to sell to the Underwriter the Additional Shares, and the Underwriter shall have the right to purchase up to 900,000 Additional Shares at the Purchase Price. The Underwriter may exercise this right in whole or from time to time in part by giving written notice not later than 30 days after the date of this Agreement. Any exercise notice shall specify the number of Additional Shares to be purchased by the Underwriter and the date on which such shares are to be purchased. Each purchase date must be at least two business days after the written notice is given and may not be earlier than the closing date for the Firm Shares nor later than ten business days after the date of such notice. Additional Shares may be purchased as provided in Section 4 hereof solely for the purpose of covering over-allotments made in connection with the offering of the Firm Shares. On each day, if any, that Additional Shares are to be purchased (an “Option Closing Date”), the Underwriter agrees to purchase the Additional Shares.

The Company hereby agrees that, without the prior written consent of the Underwriter, it will not, during the period ending 90 days after the date of the Prospectus, (i) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend, or otherwise transfer or dispose of, directly or indirectly, any shares of Common Stock or any securities convertible into or exercisable or exchangeable for Common Stock or (ii) enter into any swap or

other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of the Common Stock, whether any such transaction described in clause (i) or (ii) above is to be settled by delivery of Common Stock or such other securities, in cash or otherwise. The foregoing sentence shall not apply to (A) the Shares to be sold hereunder, (B) the issuance by the Company of shares of Common Stock upon the exercise of an option or warrant or the conversion of a security outstanding on the date hereof of which the Underwriter has been advised in writing, (C) the grant of options to purchase Common Stock under the Company’s stock option plans or (D) the issuance by the Company of shares of Common Stock under the Company’s employee stock purchase plan.

3. Terms of Public Offering. The Company is advised that the Underwriter proposes to make a public offering of the Shares as soon after the Registration Statement and this Agreement have become effective as in the Underwriter’s judgment is advisable. The Company is further advised by the Underwriter that the Shares are to be offered to the public initially at $9.63 a share (the “Public Offering Price”) and to certain dealers selected by the Underwriter at a price that represents a concession not in excess of $.35 a share under the Public Offering Price.

4. Payment and Delivery. Payment for the Firm Shares shall be made to the Company in Federal or other funds immediately available in New York City against delivery of such Firm Shares for the account of the Underwriter at 10:00 a.m., New York City time, on June 11, 2003 or at such other time on the same or such other date, not later than June 18, 2003 as shall be designated in writing by the Underwriter. The time and date of such payment are hereinafter referred to as the “Closing Date.”

Payment for any Additional Shares shall be made to the Company in Federal or other funds immediately available in New York City against delivery of such Additional Shares for the account of the Underwriter at 10:00 a.m., New York City time, on the date specified in the corresponding notice described in Section 2 or at such other time on the same or on such other date, in any event not later than July 11, 2003 as shall be designated in writing by the Underwriter.

The Firm Shares and Additional Shares shall be registered in such names and in such denominations as the Underwriter shall request in writing not later than one full business day prior to the Closing Date or the applicable Option Closing Date, as the case may be. The Firm Shares and Additional Shares shall be delivered to the Underwriter on the Closing Date or an Option Closing Date, as the case may be, for the account of the Underwriter, with any transfer taxes payable in connection with the transfer of the Shares to the Underwriter duly paid, against payment of the Purchase Price therefor.

5. Conditions to the Underwriter’s Obligations. The obligations of the Company to sell the Shares to the Underwriter and the obligation of the

Underwriter to purchase and pay for the Shares on the Closing Date are subject to the following conditions:

(a) Subsequent to the execution and delivery of this Agreement and prior to the Closing Date:

(i) there shall not have occurred any downgrading, nor shall any notice have been given of any intended or potential downgrading or of any review for a possible change that does not indicate the direction of the possible change, in the rating accorded any of the Company’s securities by any “nationally recognized statistical rating organization,” as such term is defined for purposes of Rule 436(g)(2) under the Securities Act; and

(ii) there shall not have occurred any change, or any development involving a prospective change, in the condition, financial or otherwise, or in the earnings, business or operations of the Company, from that set forth in the Prospectus (exclusive of any amendments or supplements thereto subsequent to the date of this Agreement) that, in your judgment, is material and adverse and that makes it, in your judgment, impracticable to market the Shares on the terms and in the manner contemplated in the Prospectus.

(b) The Underwriter shall have received on the Closing Date a certificate, dated the Closing Date and signed by an executive officer of the Company, to the effect set forth in Section 5(a)(i) above and to the effect that the representations and warranties of the Company contained in this Agreement are true and correct as of the Closing Date and that the Company has complied with all of the agreements and satisfied all of the conditions on its part to be performed or satisfied hereunder on or before the Closing Date.

The officer signing and delivering such certificate may rely upon the best of his or her knowledge as to proceedings threatened.

(c) The Underwriter shall have received on the Closing Date an opinion of Cooley Godward LLP, outside counsel for the Company, dated the Closing Date, to the effect that:

(i) the Company has been duly incorporated, is validly existing as a corporation in good standing under the laws of the State of Delaware, has the corporate power and authority to own its property and to conduct its business as described in the Prospectus and, to such counsel’s knowledge, is duly qualified to transact business and is in good standing in each jurisdiction in which the conduct of its business or its ownership or leasing of

property requires such qualification, except to the extent that the failure to be so qualified or be in good standing would not have a material adverse effect on the Company;

(ii) the authorized capital stock of the Company conforms as to legal matters to the description thereof contained in the Prospectus under the caption “Description of Capital Stock;”

(iii) the shares of Common Stock outstanding prior to the issuance of the Shares have been duly authorized and are validly issued, fully paid and non-assessable;

(iv) the Shares have been duly authorized and, when issued and delivered in accordance with the terms of this Agreement, will be validly issued, fully paid and non-assessable, and the issuance of such Shares will not be subject to any preemptive rights or, to such counsel’s knowledge, similar rights;

(v) this Agreement has been duly authorized, executed and delivered by the Company;

(vi) the execution and delivery by the Company of, and the performance by the Company of its obligations under, this Agreement will not contravene any provision of applicable law or the certificate of incorporation or by-laws of the Company or, to the best of such counsel’s knowledge, any agreement or other instrument binding upon the Company that is either (A) filed as an exhibit to the Registration Statement, (B) filed as an exhibit to a document incorporated by reference in the Registration Statement or (C) which the Company has advised such counsel will be filed as an exhibit to the next periodic report on Form 10-Q and was executed by the Company between April 1, 2003 and immediately prior to the Closing, or, to the best of such counsel’s knowledge, any judgment, order or decree of any governmental body, agency or court having jurisdiction over the Company, and no consent, approval, authorization or order of, or qualification with, any governmental body or agency is required for the performance by the Company of its obligations under this Agreement, except as have been made or obtained or except such as may be required by the securities or Blue Sky laws of the various states in connection with the offer and sale of the Shares;

(vii) the statements in (A) the Base Prospectus under the captions “Description of Capital Stock” and “Plan of Distribution, (B) the Prospectus Supplement under the caption “Underwriter” and (C) the Registration Statement in Item 15, in

each case insofar as such statements constitute matters of law, summaries of legal matters, provisions of the Company’s certificate of incorporation or by-laws or other equivalent corporate governance documents, or legal proceedings, or legal conclusions, have been reviewed by such counsel and fairly present and summarize, in all material respects, the mattes referred to herein;

(viii) after due inquiry, such counsel does not know of any legal or governmental proceedings pending or overtly threatened to which the Company is a party or to which any of the properties of the Company is subject that are required to be described in the Registration Statement or the Prospectus and are not so described or of any statutes, regulations, contracts or other legal documents that are required to be described in the Registration Statement or the Prospectus or to be filed as exhibits to the Registration Statement that are not described or filed as required;

(ix) the Company is not, and after giving effect to the offering and sale of the Shares and the application of the proceeds thereof as described in the Prospectus will not be, required to register as an “investment company” as such term is defined in the Investment Company Act of 1940, as amended; and

(x) to such counsel’s knowledge, (A) each document filed pursuant to the Exchange Act and incorporated by reference in the Registration Statement and the Prospectus (except for the financial statements and financial schedules and other financial and statistical data included therein, as to which such counsel need not express any belief) complied as to form when filed in all material respects with the requirements of the Exchange Act and the applicable rules and regulations of the Commission thereunder and (B) the Registration Statement or the Prospectus (except for the financial statements and financial schedules and other financial and statistical data included therein, as to which such counsel need not express any belief) complied as to form in all material respects with the requirements of the Securities Act and the applicable rules and regulations of the Commission thereunder.

In addition, counsel shall confirm that such counsel has participated in conferences with officers and other representatives of the Company, representatives of the independent public accountants of the Company and representatives of the Underwriter at which the contents of the Registration Statement and Prospectus were discussed and, although such counsel is not passing upon and does not assume responsibility for the accuracy, completeness

or fairness of the statements contained in the Registration Statement or Prospectus (except as and to the extent stated in subparagraphs (ii) and (vii) above), on the basis of the foregoing, nothing has come to the attention of such counsel that causes such counsel to believe that (A) the Registration Statement or the prospectus included therein (except for the financial statements and financial schedules and other financial and statistical data included therein, as to which such counsel need not express any belief) at the time the Registration Statement became effective contained an untrue statement of a material fact or omitted to state a material fact required to be stated therein or necessary to make the statements therein not misleading or (B) the Prospectus (except for the financial statements and financial schedules and other financial and statistical data included therein, as to which such counsel need not express any belief) as of its date or as of the Closing Date contained or contains an untrue statement of a material fact or omitted or omits to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading.

(d) The Underwriter shall have received on the Closing Date an opinion of Davis Polk & Wardwell, counsel for the Underwriter, dated the Closing Date, covering the matters referred to in Sections 5(c)(iv), 5(c)(v), 5(c)(vii) (but only as to the statements in the Base Prospectus under “Description of Capital Stock” and under the caption “Underwriter” in the Prospectus Supplement) and 5(c)(x) above and the immediately following confirmation.

With respect to the confirmation above, Cooley Godward LLP and Davis Polk & Wardwell may state that their beliefs are based upon their participation in the preparation of the Registration Statement and Prospectus and any amendments or supplements thereto and review and discussion of the contents thereof, but are without independent check or verification, except as specified.

The opinion of Cooley Godward LLP described in Section 5(c) above shall be rendered to the Underwriter at the request of the Company and shall so state therein.

(e) The Underwriter shall have received, on each of the date hereof and the Closing Date, a letter dated the date hereof or the Closing Date, as the case may be, in form and substance satisfactory to the Underwriter, from Ernst & Young LLP, independent public accountants, containing statements and information of the type ordinarily included in accountants’ “comfort letters” to underwriters with respect to the financial statements and certain financial information contained in or incorporated by reference into the Registration Statement and the Prospectus; provided that the letter delivered on the Closing Date shall use a “cut-off date” not earlier than the date hereof.

(f) The “lock-up” agreements, each substantially in the form of Exhibit A hereto, between you and certain shareholders, officers and directors of the Company relating to sales and certain other dispositions of shares of Common Stock or certain other securities, delivered to you on or before the date hereof, shall be in full force and effect on the Closing Date.

The obligation of the Underwriter to purchase Additional Shares hereunder are subject to the delivery to you on the applicable Option Closing Date of such documents as you may reasonably request with respect to the good standing of the Company, the due authorization and issuance of the Additional Shares to be sold on such Option Closing Date and other matters related to the issuance of such Additional Shares.

6. Covenants of the Company. In further consideration of the agreements of the Underwriter herein contained, the Company covenants with the Underwriter as follows:

(a) To furnish to you, without charge, three signed copies of the Registration Statement (including exhibits thereto) and for delivery to the Underwriter a conformed copy of the Registration Statement (without exhibits thereto but including documents incorporated by reference) and to furnish to you in New York City, without charge, prior to 10:00 a.m. New York City time on the business day next succeeding the date of this Agreement and during the period mentioned in Section 6(c) below, as many copies of the Prospectus, any documents incorporated by reference and any supplements and amendments thereto or to the Registration Statement as you may reasonably request.

(b) Before amending or supplementing the Registration Statement or the Prospectus, to furnish to you a copy of each such proposed amendment or supplement and not to file any such proposed amendment or supplement to which you reasonably object, and to file with the Commission within the applicable period specified in Rule 424(b) under the Securities Act any prospectus required to be filed pursuant to such Rule.

(c) If, during such period after the first date of the public offering of the Shares as in the opinion of counsel for the Underwriter the Prospectus is required by law to be delivered in connection with sales by an Underwriter or dealer, any event shall occur or condition exist as a result of which it is necessary to amend or supplement the Prospectus in order to make the statements therein, in the light of the circumstances when the Prospectus is delivered to a purchaser, not misleading, or if, in the opinion of counsel for the Underwriter, it is necessary to amend or supplement the Prospectus to comply with applicable law, forthwith to prepare, file with the Commission and furnish, at its own expense, to the Underwriter and to the dealers (whose names and addresses you will

furnish to the Company) to which Shares may have been sold by you on behalf of the Underwriter and to any other dealers upon request, either amendments or supplements to the Prospectus so that the statements in the Prospectus as so amended or supplemented will not, in the light of the circumstances when the Prospectus is delivered to a purchaser, be misleading or so that the Prospectus, as amended or supplemented, will comply with law.

(d) To endeavor to qualify the Shares for offer and sale under the securities or Blue Sky laws of such jurisdictions as you shall reasonably request.

(e) To make generally available to the Company’s security holders and to you as soon as practicable an earnings statement covering the twelve-month period ending June 30, 2004 that satisfies the provisions of Section 11(a) of the Securities Act and the rules and regulations of the Commission thereunder.

(f) Whether or not the transactions contemplated in this Agreement are consummated or this Agreement is terminated, to pay or cause to be paid all expenses incident to the performance of its obligations under this Agreement, including: (i) the fees, disbursements and expenses of the Company’s counsel and the Company’s accountants in connection with the registration and delivery of the Shares under the Securities Act and all other fees or expenses in connection with the preparation and filing of the Registration Statement, any preliminary prospectus, the Prospectus and amendments and supplements to any of the foregoing, including all printing costs associated therewith, and the mailing and delivering of copies thereof to the Underwriter and dealers, in the quantities hereinabove specified, (ii) all costs and expenses related to the transfer and delivery of the Shares to the Underwriter, including any transfer or other taxes payable thereon, (iii) the cost of printing or producing any Blue Sky or legal investment memorandum in connection with the offer and sale of the Shares under state securities laws and all expenses in connection with the qualification of the Shares for offer and sale under state securities laws as provided in Section 6(d) hereof, including filing fees and the reasonable fees and disbursements of counsel for the Underwriter in connection with such qualification and in connection with the Blue Sky or legal investment memorandum, (iv) all filing fees and the reasonable fees and disbursements of counsel to the Underwriter incurred in connection with the review and qualification of the offering of the Shares by the National Association of Securities Dealers, Inc., (v) all costs and expenses incident to listing the Shares on the Nasdaq National Market, (vi) the cost of printing certificates representing the Shares, (vii) the costs and charges of any transfer agent, registrar or depositary, (viii) the costs and expenses of the Company relating to investor presentations on any “road show” undertaken in connection with the marketing of the offering of the Shares,

including, without limitation, expenses associated with the production of road show slides and graphics, fees and expenses of any consultants engaged in connection with the road show presentations with the prior approval of the Company, travel and lodging expenses of the representatives and officers of the Company and any such consultants, and the cost of any aircraft chartered in connection with the road show, (ix) the document production charges and expenses associated with printing this Agreement and (x) all other costs and expenses incident to the performance of the obligations of the Company hereunder for which provision is not otherwise made in this Section. It is understood, however, that except as provided in this Section, Section 7 entitled “Indemnity and Contribution”, and the last paragraph of Section 9 below, the Underwriter will pay all of their costs and expenses, including fees and disbursements of their counsel, stock transfer taxes payable on resale of any of the Shares by them and any advertising expenses connected with any offers they may make.

7. Indemnity and Contribution. (a) The Company agrees to indemnify and hold harmless the Underwriter, each person, if any, who controls any Underwriter within the meaning of either Section 15 of the Securities Act or Section 20 of the Exchange Act and each affiliate of the Underwriter within the meaning of Rule 405 under the Securities Act from and against any and all losses, claims, damages and liabilities (including, without limitation, any legal or other expenses reasonably incurred in connection with defending or investigating any such action or claim) caused by any untrue statement or alleged untrue statement of a material fact contained in the Registration Statement or any amendment thereof, or the Prospectus (as amended or supplemented if the Company shall have furnished any amendments or supplements thereto), or caused by any omission or alleged omission to state therein a material fact required to be stated therein or necessary to make the statements therein not misleading, except insofar as such losses, claims, damages or liabilities are caused by any such untrue statement or omission or alleged untrue statement or omission based upon information relating to any Underwriter furnished to the Company in writing by the Underwriter expressly for use therein provided, however, that the foregoing indemnity agreement with respect to any preliminary prospectus shall not inure to the benefit of the Underwriter from whom the person asserting any such losses, claims, damages or liabilities purchased Shares, or any person controlling such Underwriter or any affiliate of any Underwriter within the meaning of Rule 405 under the Securities Act which affiliate directly or indirectly participated in the distribution of the Shares, if a copy of the Prospectus (as then amended or supplemented if the Company shall have furnished any amendments or supplements thereto) was not sent or given by or on behalf of the Underwriter to such person, if required by law so to have been delivered, at or prior to the written confirmation of the sale of the Shares to such person, and if the Prospectus (as so amended or supplemented) would have cured the defect giving rise to such losses,

claims, damages or liabilities, unless such failure is the result of noncompliance by the Company with Section 6(a) hereof.

(b) The Underwriter agrees to indemnify and hold harmless the Company, its directors, its officers who sign the Registration Statement and each person, if any, who controls the Company within the meaning of either Section 15 of the Securities Act or Section 20 of the Exchange Act to the same extent as the foregoing indemnity from the Company to the Underwriter, but only with reference to information relating to the Underwriter furnished to the Company in writing by the Underwriter expressly for use in the Registration Statement, the Prospectus or any amendments or supplements thereto.

(c) In case any proceeding (including any governmental investigation) shall be instituted involving any person in respect of which indemnity may be sought pursuant to Section 7(a) or 7(b), such person (the “indemnified party”) shall promptly notify the person against whom such indemnity may be sought (the “indemnifying party”) in writing and the indemnifying party, upon request of the indemnified party, shall retain counsel reasonably satisfactory to the indemnified party to represent the indemnified party and any others the indemnifying party may designate in such proceeding and shall pay the fees and disbursements of such counsel related to such proceeding. In any such proceeding, any indemnified party shall have the right to retain its own counsel, but the fees and expenses of such counsel shall be at the expense of such indemnified party unless (i) the indemnifying party and the indemnified party shall have mutually agreed to the retention of such counsel or (ii) the named parties to any such proceeding (including any impleaded parties) include both the indemnifying party and the indemnified party and representation of both parties by the same counsel would be inappropriate due to actual or potential differing interests between them. It is understood that the indemnifying party shall not, in respect of the legal expenses of any indemnified party in connection with any proceeding or related proceedings in the same jurisdiction, be liable for the fees and expenses of more than one separate firm (in addition to any local counsel) for all such indemnified parties and that all such fees and expenses shall be reimbursed as they are incurred. Such firm shall be designated in writing by Morgan Stanley & Co. Incorporated, in the case of parties indemnified pursuant to Section 7(a), and by the Company, in the case of parties indemnified pursuant to Section 7(b). The indemnifying party shall not be liable for any settlement of any proceeding effected without its written consent, but if settled with such consent or if there be a final judgment for the plaintiff, the indemnifying party agrees to indemnify the indemnified party from and against any loss or liability by reason of such settlement or judgment. No indemnifying party shall, without the prior written consent of the indemnified party, effect any settlement of any pending or threatened proceeding in respect of which any indemnified party is or could have been a party and indemnity could have been sought hereunder by such indemnified party, unless such settlement includes an unconditional release of

such indemnified party from all liability on claims that are the subject matter of such proceeding.

(d) To the extent the indemnification provided for in Section 7(a) or 7(b) is unavailable to an indemnified party or insufficient in respect of any losses, claims, damages or liabilities referred to therein, then each indemnifying party under such paragraph, in lieu of indemnifying such indemnified party thereunder, shall contribute to the amount paid or payable by such indemnified party as a result of such losses, claims, damages or liabilities (i) in such proportion as is appropriate to reflect the relative benefits received by the Company on the one hand and the Underwriter on the other hand from the offering of the Shares or (ii) if the allocation provided by clause 7(d)(i) above is not permitted by applicable law, in such proportion as is appropriate to reflect not only the relative benefits referred to in clause 7(d)(i) above but also the relative fault of the Company on the one hand and of the Underwriter on the other hand in connection with the statements or omissions that resulted in such losses, claims, damages or liabilities, as well as any other relevant equitable considerations. The relative benefits received by the Company on the one hand and the Underwriter on the other hand in connection with the offering of the Shares shall be deemed to be in the same respective proportions as the net proceeds from the offering of the Shares (before deducting expenses) received by the Company and the total underwriting discounts and commissions received by the Underwriter, in each case as set forth in the table on the cover of the Prospectus, bear to the aggregate Public Offering Price of the Shares. The relative fault of the Company on the one hand and the Underwriter on the other hand shall be determined by reference to, among other things, whether the untrue or alleged untrue statement of a material fact or the omission or alleged omission to state a material fact relates to information supplied by the Company or by the Underwriter and the parties’ relative intent, knowledge, access to information and opportunity to correct or prevent such statement or omission.

(e) The Company and the Underwriter agree that it would not be just or equitable if contribution pursuant to this Section 7 were determined by pro rata allocation or by any other method of allocation that does not take account of the equitable considerations referred to in Section 7(d). The amount paid or payable by an indemnified party as a result of the losses, claims, damages and liabilities referred to in the immediately preceding paragraph shall be deemed to include, subject to the limitations set forth above, any legal or other expenses reasonably incurred by such indemnified party in connection with investigating or defending any such action or claim. Notwithstanding the provisions of this Section 7, the Underwriter shall not be required to contribute any amount in excess of the amount by which the total price at which the Shares underwritten by it and distributed to the public were offered to the public exceeds the amount of any damages that the Underwriter has otherwise been required to pay by reason of such untrue or alleged untrue statement or omission or alleged omission. No person guilty of fraudulent misrepresentation (within the meaning of Section 11(f)

of the Securities Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation. The remedies provided for in this Section 7 are not exclusive and shall not limit any rights or remedies which may otherwise be available to any indemnified party at law or in equity.

(f) The indemnity and contribution provisions contained in this Section 7 and the representations, warranties and other statements of the Company contained in this Agreement shall remain operative and in full force and effect regardless of (i) any termination of this Agreement, (ii) any investigation made by or on behalf of the Underwriter, any person controlling the Underwriter or any affiliate of the Underwriter or by or on behalf of the Company, its officers or directors or any person controlling the Company and (iii) acceptance of and payment for any of the Shares.

8. Termination. The Underwriter may terminate this Agreement by notice given by you to the Company, if after the execution and delivery of this Agreement and prior to the Closing Date (i) trading generally shall have been suspended or materially limited on, or by, as the case may be, any of the New York Stock Exchange, the American Stock Exchange, the Nasdaq National Market, the Chicago Board of Options Exchange, the Chicago Mercantile Exchange or the Chicago Board of Trade, (ii) trading of any securities of the Company shall have been suspended on any exchange or in any over-the-counter market, (iii) a material disruption in securities settlement, payment or clearance services in the United States shall have occurred, (iv) any moratorium on commercial banking activities shall have been declared by Federal or New York State authorities or (v) there shall have occurred any outbreak or escalation of hostilities, or any change in financial markets or any calamity or crisis that, in your judgment, is material and adverse and which, singly or together with any other event specified in this clause (v), makes it, in your judgment, impracticable or inadvisable to proceed with the offer, sale or delivery of the Shares on the terms and in the manner contemplated in the Prospectus.

9. Effectiveness; Defaulting Underwriter. This Agreement shall become effective upon the execution and delivery hereof by the parties hereto.

If this Agreement shall be terminated by the Underwriter, because of any failure or refusal on the part of the Company to comply with the terms or to fulfill any of the conditions of this Agreement, or if for any reason the Company shall be unable to perform its obligations under this Agreement, the Company will reimburse the Underwriter for all out-of-pocket expenses (including the fees and disbursements of their counsel) reasonably incurred by the Underwriter in connection with this Agreement or the offering contemplated hereunder.

10. Counterparts. This Agreement may be signed in two or more counterparts, each of which shall be an original, with the same effect as if the signatures thereto and hereto were upon the same instrument.

11. Applicable Law. This Agreement shall be governed by and construed in accordance with the internal laws of the State of New York.

12. Headings. The headings of the sections of this Agreement have been inserted for convenience of reference only and shall not be deemed a part of this Agreement.

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	Very truly yours,

	Cerus Corporation


	By:	/s/ Gregory W. Schafer
		Name: Gregory W. Schafer
		Title: VP Finance, CFO

Accepted as of the date hereof

Morgan Stanley & Co. Incorporated


By:	/s/ Bryan W. Andrzejewski
	Name: Bryan W. Andrzejewski
	Title: Executive Director

EXHIBIT A

FORM OF LOCK-UP LETTER

June 4, 2003

Morgan Stanley & Co. Incorporated

1585 Broadway

New York, NY 10036

Dear Sirs and Mesdames:

The undersigned understands that Morgan Stanley & Co. Incorporated (the “Underwriter”) proposes to enter into an Underwriting Agreement (the “Underwriting Agreement”) with Cerus Corporation, a Delaware corporation (the “Company”), providing for the public offering (the “Public Offering”) by the Underwriter, of 6,000,000 shares (the “Shares”) of the common stock, par value $.001 of the Company (the “Common Stock”).

To induce the Underwriter to continue its efforts in connection with the Public Offering, the undersigned hereby agrees that, without the prior written consent of the Underwriter, it will not, during the period commencing on the date hereof and ending 90 days after the date of the final prospectus relating to the Public Offering (the “Prospectus”), (1) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend, or otherwise transfer or dispose of, directly or indirectly, any shares of Common Stock or any securities convertible into or exercisable or exchangeable for Common Stock or (2) enter into any swap or other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of the Common Stock, whether any such transaction described in clause (1) or (2) above is to be settled by delivery of Common Stock or such other securities, in cash or otherwise. The foregoing sentence shall not apply to (a) the sale of any Shares to the Underwriter pursuant to the Underwriting Agreement, (b) transactions relating to shares of Common Stock or other securities acquired in open market transactions after the completion of the Public Offering, (c) sales by officers and directors at the Company pursuant to plans established in accordance with Rule 10b5-1 under the Securities Exchange Act of 1934, as amended (the “Exchange Act”) at a minimum limit order of $30, (d) transfers of shares of Common Stock or any security convertible into or exercisable or exchangeable for Common Stock as a bona fide gift or gifts or (e) transfers of shares of Common Stock or any security convertible into or exercisable or exchangeable for Common Stock by the undersigned to its affiliates, as such term is defined in Rule 405 under the Securities Act of 1933, as

amended; provided that in the case of any transfer or distribution pursuant to clause (d) and (e), (i) each donee or distributee shall execute and deliver to Underwriter a duplicate form of this Lock-up Letter and (ii) no filing by any party (donor, donee, transferor or transferee) under Section 16(a) of the Securities Exchange Act of 1934, as amended, shall be required or shall be made voluntarily in connection with such transfer or distribution (other than a filing on a Form 5 made after the expiration of the 90-day period referred to above). In addition, the undersigned agrees that, without the prior written consent of the Underwriter, it will not, during the period commencing on the date hereof and ending 90 days after the date of the Prospectus, make any demand for or exercise any right with respect to, the registration of any shares of Common Stock or any security convertible into or exercisable or exchangeable for Common Stock. The undersigned also agrees and consents to the entry of stop transfer instructions with the Company’s transfer agent and registrar against the transfer of the undersigned’s shares of Common Stock except in compliance with the foregoing restrictions.

The undersigned understands that the Company and the Underwriter are relying upon this Lock-Up Agreement in proceeding toward consummation of the Public Offering. The undersigned further understands that this Lock-Up Agreement is irrevocable and shall be binding upon the undersigned’s heirs, legal representatives, successors and assigns.

Whether or not the Public Offering actually occurs depends on a number of factors, including market conditions. Any Public Offering will only be made pursuant to an Underwriting Agreement, the terms of which are subject to negotiation between the Company and the Underwriter.

	Very truly yours,


	(Name)


	(Address)