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     FIRST AMENDMENT TO SUBLEASE

Dated:    July      , 1996

     WHEREAS, by a Sublease (the "Sublease") dated as of August 31, 1995,
IMMUNOGEN, INC. ("Landlord") sublet unto ASTRA RESEARCH CENTER BOSTON, INC. a
Massachusetts corporation ("Tenant"), certain Premises (more particularly
defined in the Sublease), on the terms and conditions set forth therein. The
Premises demised under the Sublease comprise a portion of the building known as
and numbered 128 Sidney Street, Cambridge, Massachusetts (the "Building"), which
portion contains approximately 30,778 rentable square feet (consisting of 30,309
square of office and laboratory space and 469 square feet of shipping area), and
constitutes the entire Building other than the portion shown on Exhibit A to the
Main Lease, which portion (the "Retained Portion") was retained by Landlord for
its use; and

     WHEREAS, Landlord and Tenant now desire to amend the Sublease by, among
other things, expanding the Premises by approximately 6,453 square feet (the
"Additional Space"), so as to comprise the entire Building and Land (as defined
in the Sublease), other than certain loading and office areas, which will
continue to be reserved to the Landlord's use, and amending the right of Tenant
to extend the term of the Sublease.

     NOW, therefore, in consideration of the foregoing, and for other good and
valuable consideration, each to the other paid, Landlord and Tenant agree that
the Sublease be amended as follows:

     1.   Effective as of the date (the "Effective Date") on which Landlord
          tenders possession of the Additional Space to Tenant, the description
          of the Premises, as set forth in Section 1.0 of the Sublease shall be
          amended to read as follows:

          "The entire building known as and numbered 128 Sidney Street,
          Cambridge, Massachusetts (the "Building"), which contains
          approximately 37,700 rentable square feet; excepting, however, and
          reserving to Landlord (i) that portion of the loading area of the
          Building shown on Exhibit A-1, which portion contains approximately
          469 square feet of space, thus leaving the Tenant 37,231 square feet
          of space, and (ii) a portion of office area. The Building is located
          on the parcel of land (the "Land") shown on the plan attached hereto
          as Exhibit B, together with the portion(s) of the parking area located
          on the Land as are hereafter designated by Landlord and Tenant
          pursuant to Section 9.1 below (collectively, the "Leased Land") (such
          portion of the Building being sometimes hereinafter referred to as the
          "Premises")."

     2.   (a) Effective as of the date hereof, Landlord and Tenant confirm that
          Tenant has duly exercised its options to extend the term of the
          Sublease, as described in Section 2.2 thereof, for both the First
          Extended Term and the Second Extended Term, such that the term of the
          Sublease will expire on February 28, 1999. In addition to the First
          and Second Extended Terms, and provided that, at the time of each such
          exercise, (i) there exists no Event of Default under the Sublease;
          (ii) the Sublease is still in full force and effect; and (iii) Tenant
          shall not have assigned the Sublease or sublet any or all of the
          Premises, Landlord agrees that Tenant shall have the right to further
          extend the term of the Sublease for two additional 


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          extended terms (the "Third Extended Term" and the "Fourth Extended
          Term," respectively) of six (6) months each. The Third Extended Term
          shall commence on March 1, 1999, and shall end on August 31, 1999, and
          the Fourth Extended Term shall commence on September 1, 1999, and
          shall end on February 28, 2000. Tenant shall exercise each such option
          by giving Landlord notice of its desire to do so, not later than (i)
          August 31, 1997 with respect to the Third Extended Term, and (ii)
          February 28, 1998 with respect to the Fourth Extended Term, it being
          agreed that time shall be of the essence with respect to the giving of
          each such notice. The giving of any such notice shall automatically
          extend the term of this Sublease for the applicable Extended Term, and
          no instrument of renewal need be executed. In the event that Tenant
          fails to give such notice to Landlord, the term of this Lease shall
          automatically terminate at the end of the term then in effect, and
          Tenant shall have no further right or option to extend the term of
          this Sublease. Each Extended Term shall be on all the terms and
          conditions of this Lease, except that the Rent for each Extended Term
          shall be determined in accordance with Section 3 below.

     3.   (a) Tenant shall continue to pay Rent on the existing portion of the
          Premises as outlined in the Sublease, which is currently $19.50 per
          square foot. In addition, commencing on the Effective Date, Tenant
          shall pay rent on the Additional Space at the rate of $6.55 per square
          foot per annum. For example, for the period commencing on the
          Effective Date and expiring on the last day of the Initial Term, Rent
          shall be $642,438.15 ($19.50 x 30,778 square feet contained in the
          existing Premises, plus $6.55 x 6,453 square feet of Additional Space)
          per annum. Thereafter, the Rent for each portion of the Premises shall
          be separately adjusted as provided in the Sublease, as amended hereby,
          and the sum of the adjusted rents shall be payable.

          (b) The Rent for the Third Extended Term shall be an annualized rate
          equal to the product of (x) the annual Rent payable with respect to
          the Second Extended Term, multiplied by (y) a fraction, the numerator
          of which is the point at which the Index stood at the last day of the
          Second Extended Term, and the denominator of which is the point at
          which the Index stood at the first day of the Second Extended Term.
          The Rent for the Fourth Extended Term shall be an annualized rate
          equal to the product of (x) the annual Rent payable with respect to
          the Third Extended Term, multiplied by (y) a fraction, the numerator
          of which is the point at which the Index stood at the last day of the
          Third Extended Term, and the denominator of which is the point at
          which the Index stood at the first day of the Third Extended Term.

          (c) From and after the Effective Date, pursuant to Section 3.1 of the
          Sublease, Landlord hereby designates the following address as the
          place to which all payments shall be made to Landlord:

                    Immunogen, Inc.
                    333 Providence Highway
                    Norwood, MA 02062
                    Attn: Mr. Frank Pocher

     4.   (a) Effective as of the Effective Date, and notwithstanding the
          provisions of Section 5.1 of the Sublease to the contrary, Tenant
          shall be responsible for, and pay as provided therein, 100% of all
          taxes, special or general assessments, water rents, betterments, rates
          and charges, sewer rents and other impositions and charges described
          therein, and the definition of "Tenant's Tax Share" shall be amended
          to be "100%."


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          (b) For purposes of Section 5.3 of the Sublease, it shall be deemed
          that Landlord will not apply for any abatement of Taxes, unless
          Landlord gives Tenant notice that Landlord will do so, which notice is
          given not less than seven (7) days prior to the date on which
          applications are due.

     5.   Effective as of the Effective Date, and notwithstanding the provisions
          of Section 6.0 of the Sublease to the contrary, Tenant shall be
          responsible for, and pay as provided therein, 100% of all utilities
          described therein, including without limitation water and sewer
          charges." In addition, effective as of the Effective Date, and
          notwithstanding the provisions of Section 7.2 of the Sublease to the
          contrary, Tenant shall be responsible for, and pay as provided
          therein, 100% of all insurance premiums incurred by Landlord for the
          coverages described in such Section.

     6.   Landlord presently intends to deliver the Additional Space on or about
          September 1, 1996, but Landlord shall have no liability for failure to
          so deliver the Additional Space on such date. The Additional Space
          shall be delivered to Tenant "broom clean," and free of Landlord's
          equipment. Landlord warrants to Tenant that the HVAC equipment in the
          Additional Space is currently in good working order and condition. In
          all other respects, the Additional Space is to be delivered in its
          then AS IS condition, without representation or warranty by Landlord.
          Landlord shall not be required to perform any work in or to the
          Additional Space, or pay any allowance, to make the same ready for
          Tenant's occupancy. As to the portion of the Premises that will
          continue to be used by Landlord, Landlord and Tenant shall in good
          faith cooperate with each other to develop working procedures such
          that each party can have useful and regular access to its portion of
          such portion as it is entitled to use.

     7.   Tenant shall promptly deliver to Landlord and Overlandlord an amended
          Certificate of Insurance reflecting the increase of the size of the
          Premises pursuant to the provisions of this Amendment.

     8.   From and after the Effective Date, Landlord hereby designates the
          following address as the place to which all notices to Landlord shall
          be sent:

                    Immunogen, Inc.
                    333 Providence Highway
                    Norwood, MA 02062
                    Attn: Mr. Frank J.Pocher

     9.   From and after the Effective Date, and notwithstanding the provisions
          of Section 18.3 of the Lease to contrary, Tenant agrees that it shall
          be responsible for 100% of all amounts paid by Landlord to
          Overlandlord in respect of services provided by Overlandlord under the
          Main Lease.

     10.  Any holding over by Tenant in the Premises beyond the expiration of
          the Third or Fourth Extended Terms shall be treated under clause (x)
          of Section 21.0 of the Sublease.

     11.  Tenant hereby represents and warrants to Landlord that: (i) the
          Sublease is in full force and effect; (ii) there currently exists no
          default (or claim on which a default could be based) on the part of
          Landlord under the Sublease; (iii) the individual(s) executing this
          Amendment on behalf of Tenant is (are) acting according to direction
          of the Board of Directors of Tenant, and has (have) been duly
          authorized 


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          and directed to execute and deliver this Amendment; and (iv) no
          approval or consent of any third party or parties (other than
          Overlandlord) is required in order for Tenant to be bound by the terms
          and conditions hereof.

     12.  Any capitalized terms not defined herein shall have the meanings
          ascribed in the Sublease or the Main Lease.

     13.  Effective from and after the Effective Date, the Sublease shall be
          amended by deleting Exhibit A thereto, and substituting therefor
          Exhibit A-1 annexed hereto.

     14.  Reference is made to that certain Equipment Use and Services
          Agreement, between Landlord and Tenant, of even date with the Sublease
          (the "Equipment Agreement"). For purposes of clarification, the
          provisions of Section 2.0 of the Equipment Agreement shall be deemed
          to refer to the term of the Sublease, as affected by this Amendment.
          If the term of the Sublease is extended beyond February 28, 1999,
          Tenant will pay Rent under the Equipment Agreement as follows: The
          Basic Rent for the Third Extended Term shall be an annualized rate
          equal to the product of (x) the annual Rent payable with respect to
          the Second Extended Term, multiplied by (y) a fraction, the numerator
          of which is the point at which the Index stood at the last day of the
          Second Extended Term, and the denominator of which is the point at
          which the Index stood at the first day of the Second Extended Term.
          The Rent for the Fourth Extended Term shall be an annualized rate
          equal to the product of (x) the annual Rent payable with respect to
          the Third Extended Term, multiplied by (y) a fraction, the numerator
          of which is the point at which the Index stood at the last day of the
          Third Extended Term, and the denominator of which is the point at
          which the Index stood at the first day of the Third Extended Term.

     15.  Except as herein specifically amended, the Sublease and the Equipment
          Agreement are ratified and confirmed.


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     WITNESS the execution hereof in multiple counterparts under seal the day
     and year first above written.

     Landlord:                     IMMUNOGEN, INC.


                                   By:
                                      Name:
                                      Title:
                                      Hereunto duly authorized

                                      Date:



     Tenant:                       ASTRA RESEARCH CENTER BOSTON, INC.

                                   By:
                                      Name:
                                      Title:
                                      Hereunto duly authorized

                                      Date:


APPROVED:


MASSACHUSETTS INSTITUTE OF TECHNOLOGY


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