EXHIBIT 99.1


                                  CERTIFICATION


            PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002


        (SUBSECTIONS (a) AND (b) OF SECTION 1350, CHAPTER 63 OF TITLE 18,
                               UNITED STATES CODE)


         Pursuant to Section 906 of the Sarbanes-Oxley Act of 2002 (Subsections
(a) and (b) of Section 1350, Chapter 63 of Title 18, United States Code), each
of the undersigned officers of ImmunoGen, Inc., a Massachusetts corporation (the
"Company"), does hereby certify, to such officer's knowledge, that:

         The Quarterly Report on Form 10-Q for the quarter ended September 30,
2002 (the "Form 10-Q") of the Company fully complies with the requirements of
Section 13(a) or 15(d) of the Securities Exchange Act of 1934, and the
information contained in the Form 10-Q fairly presents, in all material
respects, the financial condition and results of operations of the Company.


Dated: November 12, 2002     /s/ Mitchel Sayare
                             ---------------------------------------------------
                             Mitchel Sayare, Chairman of the Board of Directors,
                             Chief Executive Officer and President
                             [principal executive officer]



Dated: November 12, 2002     /s/ Gregg D. Beloff
                             ---------------------------------------------------
                             Gregg D. Beloff, Chief Financial Officer and
                             Vice President, Finance
                             [principal financial officer]


         The foregoing certification is being furnished solely pursuant to
Section 906 of the Sarbanes-Oxley Act of 2002 (Subsections (a) and (b) of
Section 1350, Chapter 63 of Title 18, United States Code) and is not being filed
as part of a separate disclosure document.