EXHIBIT 5.1


              MINTZ, LEVIN, COHN, FERRIS, GLOVSKY AND POPEO, P.C.

                              One Financial Center
                          Boston, Massachusetts 02111


                                                                617 542 6000
                                                                617 542 2241 FAX


                                             December 17, 2001

ImmunoGen, Inc.
128 Sidney Street
Cambridge, Massachusetts 02139

Ladies and Gentlemen:

     We have acted as counsel to ImmunoGen Inc., a Massachusetts corporation
(the "Company"), in connection with the preparation and filing with the
Securities and Exchange Commission of a Registration Statement on Form S-8 (the
"Registration Statement"), pursuant to which the Company is registering the
issuance of a total of Two Million Five Hundred Thousand (2,500,000) shares (the
"Shares") of its common stock, $.01 par value per share, (the "Common Stock")
under the Securities Act of 1933, as amended. This opinion is being rendered in
connection with the filing of the Registration Statement. All capitalized terms
used herein and not otherwise defined shall have the respective meanings given
to them in the Registration Statement.

     In connection with this opinion, we have examined the Company's Restated
Articles of Organization and By-Laws, both as currently in effect; such other
records of the corporate proceedings of the Company and certificates of the
Company's officers as we have deemed relevant; and the Registration Statement
and the exhibits thereto.

     In our examination, we have assumed the genuineness of all signatures, the
legal capacity of natural persons, the authenticity of all documents submitted
to us as originals, the conformity to original documents of all documents
submitted to us as certified or photostatic copies and the authenticity of the
originals of such copies.

     Based upon the foregoing, we are of the opinion that (i) the Shares have
been duly and validly authorized by the Company and (ii) the Shares, when sold,
will have been duly and validly issued, fully paid and non-assessable shares of
the Company.

     Our opinion is limited to the Business Corporation Laws of the Commonwealth
of Massachusetts, and we express no opinion with respect to the laws of any
other jurisdiction. No opinion is expressed herein with respect to the
qualification of the Shares under the securities or blue sky laws of any state
or any foreign jurisdiction.


                   BOSTON NEW YORK RESTON WASHINGTON NEW HAVEN





MINTZ, LEVIN, COHN, FERRIS, GLOVSKY AND POPEO, P.C.


December 17, 2001
Page 2



     We understand that you wish to file this opinion as an exhibit to the
Registration Statement, and we hereby consent thereto.

                                          Very truly yours,




                                          MINTZ, LEVIN, COHN, FERRIS,
                                          GLOVSKY AND POPEO, P.C.