UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of

the Securities Exchange Act of 1934 (Amendment No.           )

Filed by the Registrant ☒
Filed by a Party other than the Registrant ☐
Check the appropriate box:
☐	Preliminary Proxy Statement
☐	Confidential, for Use of the Commission Only (as permitted by Rule 14a‑6(e)(2))
☐	Definitive Proxy Statement
☒	Definitive Additional Materials
☐	Soliciting Material under §240.14a‑12

ImmunoGen, Inc.
(Name of Registrant as Specified In Its Charter)
(Name of Person(s) Filing Proxy Statement, if other than the Registrant)
Payment of Filing Fee (Check the appropriate box):
☒	No fee required.
☐	Fee computed on table below per Exchange Act Rules 14a‑6(i)(1) and 0-11.
	(1)	Title of each class of securities to which transaction applies:
	(2)	Aggregate number of securities to which transaction applies:
	(3)	Per unit price or other underlying value of transaction computed pursuant to Exchange Act Rule 0‑11 (set forth the amount on which the filing fee is calculated and state how it was determined):
	(4)	Proposed maximum aggregate value of transaction:
	(5)	Total fee paid:
☐	Fee paid previously with preliminary materials.
☐	Check box if any part of the fee is offset as provided by Exchange Act Rule 0-11(a)(2) and identify the filing for which the offsetting fee was paid previously. Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing.
	(1)		Amount Previously Paid:
	(2)		Form, Schedule or Registration Statement No.:
	(3)		Filing Party:
	(4)		Date Filed:

Before You Vote How to Access the Proxy Materials Have the information that is printed in the box marked by the arrow XXXX XXXX XXXX XXXX (located on the by the arrow XXXX XXXX XXXX XXXX (located on the following page) in the subject line. How To Vote Please Choose One of the Following Voting Methods marked by the arrow XXXX XXXX XXXX XXXX (located on the following page) available and follow the instructions. E42627-P01243 Vote In Person: Many shareholder meetings have attendance requirements including, but not limited to, the possession of an attendance ticket issued by the entity holding the meeting. Please check the meeting materials for any special requirements for meeting attendance. At the meeting, you will need to request a ballot to vote these shares. Vote By Internet: To vote now by Internet, go to www.proxyvote.com. Have the information that is printed in the box Vote By Mail: You can vote by mail by requesting a paper copy of the materials, which will include a proxy card. Proxy Materials Available to VIEW or RECEIVE: NOTICE AND PROXY STATEMENTFORM 10-K How to View Online: following page) and visit: www.proxyvote .com. How to Request and Receive a PAPER or E-MAIL Copy: If you want to receive a paper or e-mail copy of these documents, you must request one. There is NO charge for requesting a copy. Please choose one of the following methods to make your request: 1) BY INTERNET:www.proxyvote.com 2) BY TELEPHONE: 1-800-579-1639 3) BY E-MAIL*:sendmaterial@proxyvote.com * If requesting materials by e-mail, please send a blank e-mail with the information that is printed in the box marked Requests, instructions and other inquiries sent to this e-mail address will NOT be forwarded to your investment advisor. Please make the request as instructed above on or before June 6, 2018 to facilitate timely delivery.

The Board of Directors recommends you vote FOR the following: 2. Election of Directors Nominees: 01) Stephen C. McCluski 02) Richard J. Wallace 03) Mark Goldberg, MD 04) Dean J. Mitchell 05) Kristine Peterson 06) Mark J. Enyedy 07) Stuart A. Arbuckle The Board of Directors recommends you vote FOR the following proposals: 1. To fix the number of members of the Board of Directors at seven (7). 3. To approve the 2018 Employee, Director and Consultant Equity Incentive Plan. 4. To approve the Employee Stock Purchase Plan. 5. To approve, on an advisory basis, the compensation paid to our named executive officers, as disclosed in our proxy statement. 6. To ratify the appointment of Ernst & Young LLP as our independent registered public accounting firm for the year ending December 31, 2018. In their discretion, the proxies are authorized to vote upon such other business as may properly come before the meeting or any adjournments thereof. E42628-P01243 Voting Items

[LOGO] E42629-P01243