AMENDMENT TO LEASE Agreement made this 31st day of August, 1995 between Massachusetts Institute of Technology, Treasurer's Office, Suite 200, 238 Main Street, Cambridge, MA 02142 ("Lessor") and ImmunoGen, Inc., 128 Sidney Street, Cambridge, MA 02139...

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                                                                Exhibit 10.26

                              AMENDMENT TO LEASE
                                       
     Agreement made this 31st day of August, 1995 between Massachusetts
Institute of Technology, Treasurer's Office, Suite 200, 238 Main Street,
Cambridge, MA 02142 ("Lessor") and ImmunoGen, Inc., 000 Xxxxxx Xxxxxx,
Xxxxxxxxx, XX 00000 ("Lessee").  Unless otherwise defined herein capitalized
terms shall have the meaning set forth in the Lease as defined below.

     WHEREAS, Lessor and Lessee are parties to a lease agreement for the land
and building known as 000 Xxxxxx Xxxxxx, Xxxxxxxxx, Xxxxxxxxxxxxx dated
December 23, 1992 (the "Lease");

     WHEREAS, Lessor and Lessee desire to amend the Lease to by deleting
Lessee's right to borrow from Lessor the cost of certain improvements made to
the Premises by the Lessee and to increase Lessee's share of profit from a
sublease of the Premises to take into account the Lessee's investment in such
improvements;

NOW, THEREFORE, the parties agree as follows:

     1.   Section 3.1(b) is revised to read as follows:

          (b)  All other costs, charges, or expenses which
               Lessee in this Lease agrees to pay or which Lessor pays or
               incurs as the result of a default by Lessee hereunder, including
               any penalty or interest which may be added for nonpayment or
               late payment thereof as provided in this Lease (collectively,
               "Additional Rent").
               
     2.   Section 8.0(c) is revised to read as follows:
               
          (c)  If Lessor consents to an assignment or sublease:
               (i) Lessee shall promptly deliver to Lessor a fully executed
               copy of said assignment or sublease; (ii) Lessee shall remain
               primarily liable to Lessor hereunder (which liability shall be
               joint and several with the assignee or sublessee); and (iii) if
               the aggregate rent and other amounts payable to Lessee under or
               in connection with such assignment or sublease, after deduction
               of the cost of Lessee's Work amortized on a straight-line basis
               over the Initial Lease Term, and the costs reasonably incurred
               by Lessee in entering into such assignment or sublease
               (including, without limitation, reasonable attorneys' fees and
               expenses, brokerage commissions, and alterations costs amortized
               on a straight-line basis over the term of such sublease),
               exceeds the Rent payable hereunder, Lessee shall pay to Lessor,
               as Additional Rent, one-half (1/2) of the amount of such excess
               immediately upon receipt thereof by Lessee.
               
               The parties agree that for the purposes of this
               Section 8.0(c) the cost of Lessee's Work is $8.91 per rentable
               square foot, but for purposes of this sublease of even date to
               be entered into by Lessee, as sublessor and ASTRA Research
               Center Boston, Inc., as sublessee, the cost of Lessee's Work
               shall be capped at $5.95 per rentable square foot, which will
               result in there being due to Lessor as Additional Rent during
               the term of such sublease an amount equal to $3.50 per rentable
               square foot during the initial term thereof and not less than
               $3.50 per rentable square foot during the extension term thereof
               (if said sublease is so extended).





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     3.   Section 13.5(b) is revised to read as follows:
               
          (b)  Lessee shall diligently act to repair and restore
               the remainder of the Premises, including without limitation,
               "Lessee's Work" (as defined in the Work Letter), and all
               Alterations made by Lessee (or, in the case of a taking, what
               remains thereof), to substantially the condition in which they
               existed prior to the occurrence of such taking or casualty,
               PROVIDED, HOWEVER, that:  (i) Lessee shall not be required to
               replicate Lessee's Work and all Alterations as they existed
               immediately prior to such casualty, but the plans and
               specifications for Lessee's repair and restoration shall be
               subject to Lessor's reasonable approval as provided in the Work
               Letter; (ii) the provisions of Sections 1, 2, 3, 4, 5, 6 and 8
               of the Work Letter shall apply to such repair; (iii) in the
               event that the amount of insurance proceeds available for the
               repair and restoration work of both Lessor and Lessee hereunder
               from the insurance policy maintained by Lessee pursuant to
               Section 7.2 above is reduced by reason of the fact that Lessor
               has maintained insurance with respect to the Premises concurrent
               in coverage with the insurance maintained by Lessee pursuant to
               Section 7.2, Lessor shall provide the funds necessary to make up
               such deficit; and (iv) promptly upon completion of its work
               under this clause (b), Lessee shall diligently act to restore
               and/or replace all of Lessee's Property to substantially the
               same condition they were in prior to the occurrence of such
               taking or casualty.  Lessee shall give Lessor written notice of
               the date on which Lessee commences such repair or restoration
               work.
               
     4.   Section 27.2 is deleted in its entirety.

     5.   The Work Letter attached to the Lease as Exhibit D is revised as 
          follows:
               
          (a)  The definitions of "Lessee's Amortization Payments" and "Lessor's
               Allowance" are deleted from Paragraph 1.
               
          (b)  The first subparagraph of Paragraph 2 is revised to read as 
               follows:
                              
               Subject to the provisions of Section 18.2 of the
               Lease, Lessor is leasing the Premises to Lessee in its "as is"
               condition, and Lessor shall have no obligation to perform any
               repairs or make any improvements to the Premises in anticipation
               of Lessee's occupancy thereof.  Lessee shall perform Lessee's
               Work at Lessee's sole cost and expense.
               
          (c)  The first subparagraph of Paragraph 3 is revised to read as 
               follows:
               
               Lessee shall be solely responsible for the
               preparation and completion of all preliminary and final Working
               Drawings.  Lessee shall retain its own architects and engineers
               to prepare Working Drawings, provided that Lessor first approves
               such engineers and architects so selected by Lessee, which
               approval shall not be unreasonably withheld or delayed.  Lessee
               shall provide copies of the preliminary Working Drawings to
               Lessor, and Lessor shall provide to Lessee within fifteen (15)
               business days thereafter a list of corrections and modifications
               which Lessor requires to be made to the Working Drawings so as
               to make the Working Drawings consistent with the base Building
               systems and to 





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               provide that the Lessee's Work is of a type capable of
               being reused by a typical research and laboratory user. 
               Lessor shall also provide to Lessee within such 15-business day
               period a list of those elements of Lessee's Work which Lessee
               must remove at the expiration or earlier termination of this
               Lease.
               
          (d)  The third sentence of the second paragraph of Paragraph 3 is 
               revised to read as follows:
               
               Lessor shall also provide to Lessee within such
               15-business day period a supplementary list of those elements of
               Lessee's Work which Lessee must remove at the expiration or
               earlier termination of this Lease (if any).
          
          (e)  Paragraph 9 is deleted in its entirety.
               
     In all other respects, the terms and provisions of the Lease are hereby
ratified and confirmed and remain in full force and effect and unamended.

     EXECUTED UNDER SEAL as of the date set forth above.


     Lessor:                  MASSACHUSETTS INSTITUTE OF TECHNOLOGY



                              By:    /s/XXXXXX X. XXXXXXXX
                                  ------------------------------------------
                                     Name:  Xxxxxx X. Xxxxxxxx
                                     Title: Director of Real Estate, Associate 
                                            Treasurer
                                     Hereunto duly authorized




     Lessee:                  IMMUNOGEN, INC.



                              By:    /s/XXXXX X. XXXXXX
                                  ------------------------------------------
                                     Name:  Xxxxx X. Xxxxxx
                                     Title:   Vice President
                                     Hereunto duly authorized










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