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): January 5, 2005

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 PRINCIPAL OFFICERS; ELECTION OF DIRECTORS; APPOINTMENT OF PRINCIPAL OFFICERS.

(a)          Not applicable

(b)         and (c):

Christopher U. Missling, PhD, Chief Financial Officer and Vice President, Finance and Treasurer, will leave ImmunoGen effective January 5, 2004 to pursue other interests. On that date, Dr. Missling and the Company entered into a separation agreement. The separation agreement provides Dr. Missling severance pay in an amount equal to 3 months of his gross weekly base salary of $ 4615.39, less all applicable federal, state and local taxes and any other employment-related deductions.

Karleen M. Oberton, ImmunoGen’s Senior Corporate Controller, will serve as ImmunoGen’s principal financial and accounting officer effective immediately while ImmunoGen conducts a search for a new Chief Financial Officer. Ms. Oberton, age 35, joined ImmunoGen as its Corporate Controller in February, 2004. From August 2002 to February 2004, Ms. Oberton was a Senior Audit Manager with Ernst & Young, LLP. Prior to joining Ernst & Young, LLP, Ms. Oberton was with Arthur Andersen, LLP where she held positions of increasing responsibility from 1991 until 2002. Ms. Oberton is a Certified Public Accountant.

(c)          Not applicable

ITEM 9.01. FINANCIAL STATEMENTS AND EXHIBITS

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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

128 Sidney Street, Cambridge, MA 02139-4239                                         TEL: (617) 995-2500   FAX: (617) 995-2510

Contacts

FOR IMMEDIATE RELEASE

ImmunoGen, Inc. Announces Departure of Chief Financial Officer

CAMBRIDGE, MA, January 7, 2005 – ImmunoGen, Inc. (Nasdaq: IMGN), a biotechnology company that develops targeted anticancer products, today announced the departure of Christopher U. Missling, PhD, Chief Financial Officer and Vice President, Finance and Treasurer, to pursue other interests.

Karleen M. Oberton, ImmunoGen’s Senior Corporate Controller, will serve as the Company’s principal financial and accounting officer while ImmunoGen conducts a search for a new Chief Financial Officer. Ms. Oberton joined ImmunoGen as Corporate Controller in February, 2004. Prior to ImmunoGen, Ms. Oberton held positions of increasing responsibility at Ernst & Young, LLP, and Arthur Andersen, LLP, which she joined in 1991. Ms. Oberton is a Certified Public Accountant.

About ImmunoGen, Inc.

ImmunoGen, Inc. develops targeted anticancer biopharmaceuticals. The Company’s proprietary Tumor-Activated Prodrug (TAP) technology uses cancer-targeting engineered antibodies to deliver a potent cell-killing agent specifically to cancer cells. ImmunoGen is advancing its wholly-owned TAP compounds, huN901-DM1 and huC242-DM4. Centocor, a wholly-owned subsidiary of Johnson & Johnson, Biogen Idec, Genentech, sanofi-aventis, Millennium Pharmaceuticals, Inc., Boehringer Ingelheim, and Abgenix have licensed the right to develop or test TAP compounds to specific targets; ImmunoGen also has a broader collaboration with sanofi-aventis. Please visit www.immunogen.com for additional information on ImmunoGen.

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