EXHIBIT 2

LOCK-UP AGREEMENT

CEMPRA, INC.

Building Four Quadrangle

6340 Quadrangle Drive, Suite 100

Chapel Hill, NC 27517

STIFEL, NICOLAUS & COMPANY, INCORPORATED

LEERINK SWANN LLC

COWEN & COMPANY, LLC

c/o Stifel, Nicolaus & Company, Incorporated

One South Street, 15th Floor

Baltimore, Maryland 21202

Ladies and Gentlemen:

The undersigned refers to the proposed Underwriting Agreement (the “Underwriting Agreement”) among Cempra, Inc., a Delaware corporation (the “Company”), Stifel, Nicolaus & Company, Incorporated, Leerink Swann LLC, Cowen & Company, LLC (the “Representatives”) and the several underwriters named therein (the “Underwriters”).  As an inducement to the Underwriters to execute the Underwriting Agreement in connection with the proposed public offering of shares of the Company’s common stock, par value $[0.001] per share (“Common Stock”), pursuant to a Registration Statement on Form S-1, the undersigned hereby agrees that from the date hereof and until 180 days after the public offering date set forth on the final prospectus used to sell the Common Stock (the “Public Offering Date”) pursuant to the Underwriting Agreement (such 180-day period being referenced to herein as the “Lock-Up Period”), to which you are or expect to become parties, the undersigned will not (and will cause any spouse or immediate family member of the spouse or the undersigned living in the undersigned’s household, any partnership, corporation or other entity within the undersigned’s control, and any trustee of any trust that holds Common Stock or other securities of the Company for the benefit of the undersigned or such spouse or family member not to) offer, sell, contract to sell (including any short sale), pledge, hypothecate, establish an open “put equivalent position” within the meaning of Rule 16a-1(h) under the Securities Exchange Act of 1934, as amended, grant any option, right or warrant for the sale of, purchase any option or contract to sell, sell any option or contract to purchase, or otherwise encumber, dispose of or transfer, or grant any rights with respect to, directly or indirectly, any shares of Common Stock or securities convertible into or exchangeable or exercisable for any shares of Common Stock, enter into a transaction which would have the same effect, or enter into any swap, hedge or other arrangement that transfers, in whole or in part, any of the economic consequences of ownership of the Common Stock, whether any such aforementioned transaction is to be settled by delivery of the Common Stock or such other securities, in cash or otherwise, or publicly disclose the intention to make any such offer, sale, pledge or disposition, or  to enter into any such transaction, swap, hedge or other arrangement, without, in each case, the prior written consent of

Stifel, Nicolaus & Company, Incorporated (“Stifel Nicolaus”) on behalf of the Representatives, which consent may be withheld in Stifel Nicolaus’ sole discretion; provided, however, that if (i) during the last 17 days of the initial Lock-Up Period, the Company releases earnings results or material news or a material event relating to the Company occurs or (ii) prior to the expiration of the initial Lock-Up Period, the Company announces that it will release earnings results during the 16-day period beginning on the last day of the initial Lock-Up Period, then in each case the Lock-Up Period will be automatically extended until the expiration of the 18-day period beginning on the date of release of the earnings results or the occurrence of the material news or material event, as applicable, unless Stifel Nicolaus on behalf of the Representatives waives, in writing, such extension.

The undersigned hereby acknowledges and agrees that written notice of any extension of the Lock-Up Period pursuant to the previous paragraph will be delivered by Stifel Nicolaus on behalf of the Representatives to the Company (in accordance with the notice provisions of the Underwriting Agreement) and that any such notice properly delivered will be deemed to have been given to, and received by, the undersigned.  The undersigned further agrees that, prior to engaging in any transaction or taking any other action that is subject to the terms of this Agreement during the period from the date of this Agreement to and including the 34th day following the expiration of the initial Lock-Up Period, it will give notice thereof to Stifel Nicolaus on behalf of the Representatives and will not consummate such transaction or take any such action unless it has received written confirmation from Stifel Nicolaus that the Lock-Up Period (as may have been extended pursuant to the previous paragraph) has expired.

If the undersigned is an officer or director of the Company, (i) the undersigned agrees that the foregoing restrictions shall be equally applicable to any issuer-directed or “friends and family” shares of Common Stock that the undersigned may purchase in the proposed public offering; (ii) the Representatives agree that, at least three business days before the effective date of any release or waiver of the foregoing restrictions in connection with a transfer of shares of Common Stock, Stifel Nicolaus on behalf of the Representatives will notify the Company of the impending release or waiver, and (iii) the Company has agreed in the Underwriting Agreement to announce the impending release or waiver by press release through a major news service at least two business days before the effective date of the release or waiver.  Any release or waiver granted by the Representatives hereunder to any such officer or director shall only be effective two business days after the publication date of such press release.  The provisions of this paragraph will not apply if (a) the release or waiver is effected solely to permit a transfer not for consideration and (b) the transferee has agreed in writing to be bound by the same terms described in this Agreement to the extent and for the duration that such terms remain in effect at the time of the transfer.

The foregoing restrictions shall not apply to bona fide gifts by the undersigned, provided that (a) each resulting transferee of the Company’s securities executes and delivers to Stifel Nicolaus on behalf of the Representatives an agreement satisfactory to Stifel Nicolaus certifying that such transferee is bound by the terms of this Agreement and has been in compliance with the terms hereof since the date first above

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written as if it had been an original party hereto and (b) to the extent any interest in the Company’s securities is retained by the undersigned (or such spouse or family member), such securities shall remain subject to the restrictions contained in this Agreement.

The foregoing restrictions shall not apply to any shares of Common Stock that may be sold by the undersigned as a selling stockholder in the proposed public offering pursuant to the Underwriting Agreement.

In addition, the undersigned agrees that, during the period commencing on the date hereof and ending 180 days after the Public Offering Date, without the prior written consent of Stifel Nicolaus on behalf of the Representatives (which consent may be withheld in its sole discretion): (a) the undersigned will not request, make any demand for or exercise any right with respect to, the registration of any Common Stock or any security convertible into or exercisable or exchangeable for Common Stock and (b) the undersigned waives any and all notice requirements and rights with respect to the registration of any such security pursuant to any agreement, understanding or otherwise to which the undersigned is a party.

Any Common Stock received upon exercise of options, warrants or any securities convertible into or exchangeable or exercisable for Common Stock granted to the undersigned will also be subject to this Agreement.  Any Common Stock acquired by the undersigned in the open market on or after the Public Offering Date (except Common Stock acquired pursuant to a “friends and family” or directed share program) will not be subject to this Agreement.  A transfer of Common Stock to a family member or a trust for the benefit of the undersigned or a family member (including by will or intestacy) or a distribution to partners, members or shareholders of the undersigned may be made, provided the transferee agrees in writing prior to such transfer to be bound by the terms of this Agreement as if it were a party hereto.

In furtherance of the foregoing, the Company and its transfer agent and registrar are hereby authorized to (a) decline to make any transfer of shares of Common Stock if such transfer would constitute a violation or breach of this Agreement and (b) place legends and stop transfer instructions on any such shares of Common Stock owned or beneficially owned by the undersigned.

This Agreement is irrevocable and shall be binding on the undersigned and the successors, heirs, personal representatives and assigns of the undersigned.  This Agreement shall be governed by and construed in accordance with the laws of the State of New York, without regard to choice of law rules.  This Agreement shall lapse and become null and void if the Public Offering Date shall not have occurred on or before April 1, 2012.

Very truly yours, /s/ Lloyd Appel
Printed Name:		Lloyd Appel, CFO Aisling Capital II LP
Date:	10/7/11

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