UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d)

OF THE SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported): September 24, 2008

ImmunoGen, Inc.

(Exact name of registrant as specified in its charter)

830 Winter Street, Waltham, MA 02451

(Address of principal executive offices)   (Zip Code)

Registrant’s telephone number, including area code: (781) 895-0600

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

o   Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

o   Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

o   Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o   Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

ITEM 5.02 – DEPARTURE OF DIRECTORS OR CERTAIN OFFICERS; ELECTION OF DIRECTORS; APPOINTMENT OF CERTAIN OFFICERS; COMPENSATORY ARRANGEMENTS OF CERTAIN OFFICERS

(a)  Not applicable.

(b) and (c)  On September 24, 2008, the Board of Directors of ImmunoGen, Inc. (“the Company”) approved a management succession plan pursuant to which Daniel M. Junius, currently the Company’s President and Chief Operating Officer, will become President and Chief Executive Officer (“CEO”) effective January 1, 2009.  Mr. Junius was also nominated to join the Company’s Board of Directors, with this nomination to be voted upon at the Company’s upcoming annual meeting of shareholders.  Mitchel Sayare, Ph.D., currently the Company’s Chairman and CEO, will continue as the Chairman of the Board and as a director of the Company.

Additional biographical information concerning Mr. Junius is contained in the Company’s press release dated September 30, 2008, which is incorporated herein by reference.

(d)  Not applicable.

(e)  Pursuant to the above-referenced management succession plan, Dr. Sayare will continue as an employee of the Company during the period from January 1, 2009 through the date of the 2009 annual meeting of shareholders, at which time he will retire from the Company.  During this period of employment, Dr. Sayare will continue to receive his current base salary and retain all his existing employee benefits, and will be eligible to receive his bonus under the annual executive bonus program for fiscal year 2009.  From and after September 24, 2008 he will not be granted any additional stock-based awards; and from and after June 30, 2009 (a) he will not participate in the Company’s annual bonus program, (b) his employment agreement will terminate and (c) his severance agreement will terminate with respect to any change in control occurring after June 30, 2009.  The Company’s Board of Directors currently anticipates that Dr. Sayare will be nominated for re-election to the Board at the 2009 annual meeting of shareholders, and if he is re-elected, the Board will evaluate whether Dr. Sayare will continue as Chairman of the Board.

Pursuant to the management succession plan, on September 24, 2008 the Compensation Committee voted to modify the terms of Dr. Sayare’s outstanding stock options to provide that all his currently unvested stock options will continue to vest in accordance with their original vesting schedules as long as Dr. Sayare remains either an employee of the Company or a member of the Board of Directors.  In addition, Dr. Sayare’s outstanding stock option awards were modified to provide that all his vested options (including those that will vest in the future as described above) will generally be exercisable until the earlier of (a) their original expiration date (i.e., ten years from the date of grant) or (b) the expiration of three months following the date on which Dr. Sayare ceases to be either an employee of the Company or a member of the Board of Directors.  Existing provisions in Dr. Sayare’s stock option agreements regarding exercisability of options following termination for cause or as a result of his death or disability remain unchanged.

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ITEM 9.01. FINANCIAL STATEMENTS AND EXHIBITS

(d): The following exhibit is being furnished herewith:

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

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Exhibit 99.1

Contact:

For Immediate Release

ImmunoGen, Inc. Announces CEO Succession Plan

– President and COO Daniel M. Junius to become CEO on January 1, 2009;

current CEO Mitchel Sayare to continue to serve as Chairman of the Board –

WALTHAM, MA, September 30, 2008 – ImmunoGen, Inc. (Nasdaq: IMGN) today announced that Daniel M. Junius, currently President and Chief Operating Officer (COO) of the Company, will become President and Chief Executive Officer (CEO) effective January 1, 2009 in accordance with a succession plan approved by ImmunoGen’s Board of Directors. Mr. Junius also has been nominated to join the Board, with this nomination to be voted upon at the Company’s upcoming annual meeting of shareholders. Mitchel Sayare, currently Chairman and CEO, will continue as the Chairman of the Board and as a director of the Company.

“Mitch should be proud of ImmunoGen’s accomplishments during his 22 years as CEO,” stated David Carter, lead director of the Board. “Today, the Company is gaining growing recognition as the leader in a field that has the potential to revolutionize the use of antibodies for the treatment of cancer, with three novel anticancer compounds in the clinic through its own programs and five in clinical testing through its partnerships with other innovative companies. None of this would have been possible without Mitch’s leadership and his belief in the potential of this technology.”

“ImmunoGen has come a long way under Mitch’s direction, and I look forward to working with the team of talented and dedicated employees here to take the Company to the next stage,” noted Daniel Junius, President and COO. “A key focus will be to continue to accelerate our transition from being primarily a research organization to one with strong, integrated research, clinical, regulatory, and manufacturing functions to support our advancing product pipeline.”

“ImmunoGen is in the process of becoming a Company with highly promising compounds at all stages of development,” commented Mitchel Sayare, Ph.D., Chairman and CEO. “Dan has made many important contributions to ImmunoGen since joining the Company, and I am confident his leadership will bring even greater success.”

-more-

ImmunoGen, Inc. Announces CEO Succession Plan

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Mr. Junius joined ImmunoGen in May 2005 as Senior Vice President, Finance and Chief Financial Officer (CFO). He was promoted to Executive Vice President, Finance and CFO in July 2006, and then to President and COO earlier this year. Mr. Junius had been Executive Vice President and CFO of New England Business Service, Inc. (NEBS) from 1998 until its acquisition by Deluxe Corporation in 2004. Prior to NEBS, he was Vice President and CFO of Nashua Corporation. Mr. Junius joined Nashua Corporation in 1984 and held financial management positions of increasing responsibility before becoming its CFO in 1996. He has a Masters in Management from Northwestern University’s Kellogg School of Management.

About ImmunoGen, Inc.

ImmunoGen, Inc. develops targeted anticancer therapeutics using its expertise in cancer biology, monoclonal antibodies and the creation and attachment of potent cell-killing agents. The Company’s Tumor-Activated Prodrug (TAP) technology uses antibodies to deliver one of ImmunoGen’s proprietary cell-killing agents specifically to cancer targets. In addition to the Company’s proprietary clinical pipeline, ImmunoGen collaborators Genentech, sanofi-aventis, Biogen Idec, and Biotest also are testing TAP compounds in the clinic, and a naked antibody is in clinical trials through the Company’s collaboration with sanofi-aventis.

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