<PAGE>   1
                                                                   Exhibit 10.24
                                                                   -------------


                            FOURTH AMENDMENT TO LEASE


     FOURTH AMENDMENT TO LEASE dated as of the 6th day of October, 1993 by and
between Mortimer B. Zuckerman, Edward H. Linde and David Barrett, Trustees of
Fourteen Cambridge Center Trust under Declaration of Trust dated February 4,
1982 and recorded with the Middlesex South District Registry of Deeds in Book
14707, Page 96 and not individually (hereinafter called the "Landlord") and
Biogen, Inc. (successor to Biogen Research Corp., successor to B. Leasing,
Inc.). Biogen, Inc., is the Tenant under the Lease and is (hereinafter called
"Tenant").


                                 R E C I T A L S
                     
     By lease dated October 4, 1982, as amended by First Amendment To Lease
dated January 19, 1989, by Second Amendment To Lease dated March 8, 1990 and by
Third Amendment To Lease dated September 25, 1991 (said Lease as so amended
being hereinafter called the "Lease"), Landlord did lease to Tenant and Tenant
did hire and lease from Landlord the "Site" and "Building" known as and numbered
Fourteen Cambridge Center, Cambridge, Massachusetts. The Site and the Building
are defined in Section 1.2 of the Lease and are collectively therein and herein
interchangeably called the "Demised Premises" or the "Premises".

     The Lease provides for an original Lease Term which Landlord and Tenant
acknowledge and agree is to expire on February 28, 1998 (herein sometimes called
the "Original Term").

     Pursuant to Section 3.2 of the Lease, Tenant has the right to extend the
Lease Term for three (3) successive periods of five (5) years each on the terms
and conditions set forth in said Section 3.2.

     Landlord and Tenant have now reached agreement on the present exercise of
Tenant's first, five (5) year extension option and on other modifications to the
Lease and desire to set forth the same.

     NOW, THEREFORE, in consideration of One Dollar ($1.00) and other good and
valuable consideration paid by each of the parties hereto to the other, the
receipt and sufficiency of which is hereby acknowledged, and in further
consideration of the provisions herein, Landlord and Tenant hereby agree as
follows:




<PAGE>   2


     1.   The Lease Term (also in the Lease sometimes called the "Term"), which
          but for this Amendment is scheduled to expire on February 28, 1998, is
          hereby presently extended for one (1) period of five (5) years from
          March 1, 1998 to February 28, 2003 (herein called the "First Extended
          Term") unless extended or sooner terminated in accordance with the
          provisions of the Lease (as herein amended). The present extension of
          the Lease Term for the First Extended Term shall be deemed to be the
          exercise of the first, five (5) year extension option provided for in
          Section 3.2 of the Lease, leaving only the second extension option
          (the "Second Extension Option") and third extension option (the "Third
          Extension Option") pursuant to said Section 3.2. The present extension
          of the Lease Term for the First Extended Term shall be on all of the
          same terms and conditions set forth in the Lease except as otherwise
          provided in this Amendment. All references in the Lease (as herein
          amended) to the "Term" or "Lease Term" shall mean and be references to
          the Original Term as presently extended by the First Extended Term.

     2.   (A) For the Original Term, Tenant shall continue to pay Annual Fixed
          Rent as provided in the Lease.

          (B) During the First Extended Term (being the period from March 1,
          1998 to February 28, 2003), Annual Fixed Rent shall be at the annual
          rate equal to the product of (i) $19.71 and (ii) the 67,362 square
          feet of Gross Building Area of the Building.

          (C) During the second and third extension option periods (if
          exercised), Annual Fixed Rent shall be payable by Tenant as provided
          in Section 3.2 of the Lease.

     3.   (A) Landlord and Tenant acknowledge and agree that Landlord has
          completed the North Garage and that Tenant's parking privileges
          pursuant to Section 16.5 of the Lease and Section 3 of the Second
          Amendment to Lease are being and shall be provided in the North
          Garage. The term "North Garage" as used in this Amendment and the
          Lease shall include both (i) the parking garage and other improvements
          (collectively, the "North Garage Improvements") located on the parcel
          of land known as Tract IV of the Parcel 2 Development 




                                      -2-
<PAGE>   3

          Area (the "North Garage Site") and (ii) the North Garage Site.

          Landlord and Tenant acknowledge that affiliates of Landlord and Tenant
          are concurrently herewith entering into a transaction that has as its
          objective the construction of a new building, on a site including part
          or all of the Expansion Parcel as defined in Section 16.27 of the
          Lease, to be known as Twelve Cambridge Center, to be owned by an
          affiliate of Tenant (the 12CC Owner") and occupied by Tenant, and that
          such transaction would include the execution of a parking lease
          between the 12CC Owner, as tenant, and Cambridge Center North Trust,
          an affiliate of Landlord that is the owner of the North Garage, as
          landlord, to provide parking rights for the 12CC Owner in the North
          Garage (the "12CC Parking Lease"). For the portion of the Lease Term
          prior to the Commencement Date of the 12CC Parking Lease, the monthly
          rates per vehicle for Tenant's parking privileges under this Lease
          shall be the rates provided in Section 3(b) of the Second Amendment to
          Lease. For the portion of the Lease Term on and after the Commencement
          Date of the 12CC Parking Lease, Section 3(b) of the Second Amendment
          is hereby amended so that the monthly rates per vehicle for Tenant's
          parking charges shall be as follows:

          (i)  Period                             Monthly Rate Per Car

               From the Commencement Date         $105.00, subject to escalation
               of the 12CC Parking Lease          as provided in Sections 3(B) 
               through December 31, 1999          and 3(C) below and subject to 
                                                  the limitation as provided in
                                                  Section 3(D) below.
                                                                                
                                                                                

               From January 1, 2000 through       $140.00, subject to escalation
               February 28, 2003                  as provided in Sections 3(B) 
                                                  and 3(C) below and subject to 
                                                  the limitation as provided in 
                                                  Section 3(D) below.



                                      -3-
<PAGE>   4
                                                                                
                                                                                

          (ii) Further, if Tenant shall exercise its Second Extension Option
               pursuant to Section 3.2 of the Lease, then the monthly rates per
               vehicle for Tenant's parking charges shall be as follows:




                                      -4-
<PAGE>   5

                Period                            Monthly Rate Per Car
                ------                            --------------------

                From March 1, 2003 through        $140.00, subject to escalation
                December 31, 2004                 as provided in Sections 3(B) 
                                                  and 3(C) below.

                From January 1, 2005 through      $180.00, subject to escalation
                February 28, 2008                 as provided in Sections 3(B) 
                                                  and 3(C) below.               

          (iii) In addition, if Tenant shall exercise its Third Extension Option
                pursuant to Section 3.2 of the Lease, then the monthly rates per
                vehicle for Tenant's parking charges shall be as follows:

                Period                            Monthly Rate Per Car
                ------                            --------------------

                From March 1, 2008 through        $180.00, subject to escalation
                December 31, 2009                 as provided in Sections 3(B) 
                                                  and 3(C) below.
                                                                               
                                                                               

                From January 1, 2010 through      $215.00, subject to escalation
                February 28, 2013                 as provided in Sections 3(B) 
                                                  and 3(C) below.
                                                                               
                                                                               

          (B) With reference to the real estate taxes for the North Garage
          referred to in this Section 3(B), it is agreed that terms used herein
          are defined as follows:

               (a)  "Tax Year" means the 12-month period beginning July 1 each
                    year during the Lease Term or if the appropriate
                    Governmental tax fiscal period shall begin on any date other
                    than July 1, such other date.

               (b)  "Tax Expenses for the North Garage" with respect to any Tax
                    Year means the aggregate "North Garage Real Estate Taxes"
                    (as hereinafter defined) with respect to that Tax 



                                      -5-
<PAGE>   6

                    Year, reduced by any net abatement receipts with respect to
                    that Tax Year.

               (c)  "Tax Expenses Allocable to Each Parking Space" means the
                    quotient of (i) Tax Expenses for the North Garage divided by
                    (ii) the total number of parking spaces in the North Garage,
                    being 1,170.

               (d)  "Monthly Tax Expenses Allocable to Each Parking Space" means
                    the quotient of (i) Tax Expenses Allocable to Each Parking
                    Space, divided by (ii) twelve (12).

               (e)  "North Garage Real Estate Taxes" means all taxes and special
                    assessments of every kind and nature assessed by any
                    Governmental authority on the North Garage Site or the North
                    Garage Improvements or both the North Garage Site and the
                    North Garage Improvements which the owner of the North
                    Garage shall be obligated to pay because of or in connection
                    with the ownership, leasing and operation of the North
                    Garage Site and the North Garage Improvements and reasonable
                    expenses of any proceedings for abatement of taxes. The
                    amount of special taxes or special assessments to be
                    included shall be limited to the amount of the installment
                    (plus any interest other than penalty interest payable
                    thereon) of such special tax or special assessment required
                    to be paid during the year in respect of which such taxes
                    are being determined. There shall be excluded from such
                    taxes all income, estate, succession, inheritance and
                    transfer taxes; provided, however, that if at any time
                    during the Lease Term the present system of ad valorem
                    taxation of real property shall be changed so that in lieu
                    of, or in addition to, the whole or any part of the ad
                    valorem tax on real property, there shall be assessed on the
                    owner of the North Garage a capital levy or other tax on the
                    gross income and/or parking receipts received with respect
                    to the North Garage Site or the North Garage Improvements,
                    or a Federal, State, County, Municipal, or




                                      -6-
<PAGE>   7

                    other local income, franchise, excise or similar tax,
                    assessment, levy or charge (distinct from any now in effect
                    in the jurisdiction in which the North Garage is located)
                    measured by or based, in whole or in part, upon any such
                    gross income and/or parking receipts, then any and all of
                    such taxes, assessments, levies or charges, to the extent so
                    measured or based, shall be deemed to be included within the
                    term "North Garage Real Estate Taxes" but only to the extent
                    that the same would be payable if the North Garage Site or
                    the North Garage Improvements were the only property of the
                    owner of the North Garage.

               (f)  "Base Taxes for the North Garage" means Tax Expenses for the
                    North Garage (hereinbefore defined) for fiscal tax year 1993
                    (that is the period beginning July 1, 1992 and ending June
                    30, 1993), being $269,147.00.

               (g)  "Base Taxes Allocable to Each Parking Space" means the
                    quotient of (i) Base Taxes for the North Garage divided by
                    (ii) the total number of parking spaces in the North Garage,
                    being 1,170, which quotient is $230.04 per parking space.

               (h)  "Monthly Base Taxes Allocable to Each Parking Space" means
                    the quotient of (i) "Base Taxes Allocable to Each Parking
                    Space", divided by (ii) twelve (12), which quotient is
                    $19.17 per parking space.

          If with respect to any full calendar month or fraction thereof falling
          within the Term after the Commencement Date of the 12CC Parking Lease,
          Monthly Tax Expenses Allocable to Each Parking Space for a full month
          exceed Monthly Base Taxes Allocable to Each Parking Space or for any
          such fraction of a calendar month exceed the corresponding fraction of
          Monthly Base Taxes Allocable to Each Parking Space (such amount being
          hereinafter referred to as the "North Garage Tax Excess"), then Tenant
          shall pay to Landlord, as Additional Rent, the product of (i) such
          North Garage Tax Excess and (ii) the total number automobiles for
          which Tenant has




                                      -7-
<PAGE>   8

          parking privileges in the North Garage under the terms of the Lease
          during such calendar month (the "North Garage Tax Excess Payment").
          Payments by Tenant on account of the North Garage Tax Excess Payment
          shall be made monthly at the time and in the fashion herein provided
          for the payment of Annual Fixed Rent. The amount so to be paid to
          Landlord shall be an amount from time to time reasonably estimated by
          Landlord to be sufficient to provide Landlord, in the aggregate, a sum
          equal to the North Garage Tax Excess Payment for each Tax Year during
          the Lease Term, ten (10) days at least before the day on which tax
          payments by the owner of the North Garage would become delinquent.
          Promptly after Tax Expenses Allocable to Each Parking Space are
          determinable for the first such Tax Year or fraction thereof and for
          each succeeding Tax Year or fraction thereof during the Lease Term,
          Landlord shall render to Tenant a statement in reasonable detail
          certified by a representative of Landlord showing North Garage Real
          Estate Taxes for such Tax Year or fraction thereof, abatements and
          refunds, if any, of any such taxes and assessments, expenditures
          incurred in obtaining such abatement or refund, the amount of the
          North Garage Tax Excess and the North Garage Tax Excess Payment for
          each calendar month during such Tax Year or fraction thereof, the
          amount thereof already paid by Tenant and the amount thereof overpaid
          by, or remaining due from Tenant for the period covered by such
          statement. Within thirty (30) days after the receipt of such
          statement, Tenant shall pay any sum remaining due. Any balance shown
          as due to Tenant shall be credited against Annual Fixed Rent next due,
          or refunded to Tenant if the Lease Term has then expired and Tenant
          has no further obligation to Landlord. Expenditures for reasonable
          legal fees, reasonable costs charged by affiliates of Landlord and
          other expenses incurred in obtaining an abatement or refund may be
          charged against the abatement or refund before the adjustments are
          made for the Tax Year.

          To the extent that real estate taxes shall be payable to the taxing
          authority in installments with respect to periods less than a Tax
          Year, the statement to be furnished by Landlord shall be rendered and
          payments made on account of such installments.




                                      -8-
<PAGE>   9

          (C) With reference to the operating expenses for the North Garage
          referred to in this Section 3(C), it is agreed that terms used herein
          are defined as follows:

               (a)  "Operating Expenses Allocable to Each Parking Space" means
                    the quotient of (i) "Operating Expenses for the North
                    Garage" (as hereinafter defined) divided by (ii) the total
                    number of parking spaces in the North Garage, being 1,170.

               (b)  "Monthly Operating Expenses Allocable to Each Parking Space"
                    means the quotient of (i) Operating Expenses Allocable to
                    Each Parking Space, divided by (ii) twelve (12).

               (c)  "Base Operating Expenses for the North Garage" (as defined
                    below) means Operating Expenses for the North Garage for
                    calendar year 1993 (that is the period beginning January 1,
                    1993 and ending December 31, 1993).

               (d)  "Base Operating Expenses Allocable to Each Parking Space"
                    means the quotient of (i) Base Operating Expenses for the
                    North Garage divided by (ii) the total number of parking
                    spaces in the North Garage, being, 1,170.

               (e)  "Monthly Base Operating Expenses Allocable to Each Parking
                    Space" means the quotient of (i) Base Operating Expenses
                    Allocable to Each Parking Space, divided by (ii) twelve
                    (12).

               (f)  "Operating Expenses for the North Garage" means the cost of
                    operation of the North Garage incurred by the owner of the
                    North Garage. Such costs charged by affiliates of Landlord
                    shall be limited to reasonable costs and such costs shall
                    exclude payments of debt service and any other mortgage
                    charges, brokerage commissions, salaries of executives and
                    owners not directly employed in the management or operation
                    of the North Garage and the general overhead and
                    administrative expenses of the home office of the owner of




                                      -9-
<PAGE>   10

                    the North Garage or such owner's managing agent, but shall
                    include, without limitation:

                            (i)    compensation, wages and all fringe benefits,
                                   workmen's compensation insurance premiums and
                                   payroll taxes paid to, for or with respect to
                                   all persons for their services in the
                                   operating, maintaining or cleaning of the
                                   North Garage Improvements or the North Garage
                                   Site;

                            (ii)   payments under service contracts with
                                   independent contractors for operating,
                                   maintaining or cleaning of the North Garage
                                   Improvements or the North Garage Site;

                            (iii)  steam, water, sewer, gas, oil, electricity
                                   and telephone charges and costs of
                                   maintaining letters of credit or other
                                   security as may be required by utility
                                   companies as a condition of providing such
                                   services;

                            (iv)   cost of maintenance, cleaning and repairs
                                   (other than repairs not properly chargeable
                                   against income or reimbursed from contractors
                                   under guarantees);

                            (v)    cost of snow removal and care of landscaping;

                            (vi)   cost of building and cleaning supplies and
                                   equipment;

                            (vii)  premiums for insurance carried with respect
                                   to the North Garage (including, without
                                   limitation, liability insurance, insurance
                                   against loss in case of fire or casualty and
                                   business interruption insurance and, if there
                                   be any first mortgage on the North Garage,



                                      -10-
<PAGE>   11

                                   including such insurance as may be required
                                   by the holder of such first mortgage);

                            (viii) management fees at reasonable rates
                                   consistent with the services rendered;

                            (ix)   the North Garage's share of operating
                                   expenses related to the common areas and
                                   facilities within the Parcel 2 Development
                                   Area for use of tenants of the Building in
                                   common with tenants of other buildings in the
                                   Parcel 2 Development Area;

                            (x)    depreciation for capital expenditures made by
                                   the owner of the Garage (x) to reduce
                                   Operating Expenses for the North Garage if
                                   the owner of the North Garage reasonably
                                   shall have determined that the annual
                                   reduction in Operating Expenses for the North
                                   Garage shall exceed depreciation therefor or
                                   (y) to comply with applicable laws, rules,
                                   regulations, requirements, statutes,
                                   ordinances, by-laws and court decisions of
                                   all public authorities which are now or
                                   hereafter in force (herein collectively
                                   called "Legal Requirements"), (plus, in the
                                   case of both (x) and (y), an interest factor,
                                   reasonably determined by the owner of the
                                   North Garage, as being the interest rate then
                                   charged for long term mortgages by
                                   institutional lenders on like properties
                                   within the general locality in which the
                                   North Garage is located), and in the case of
                                   both (x) and (y) depreciation shall be
                                   determined by dividing the original cost of
                                   such capital



                                      -11-
<PAGE>   12
                                   expenditure by the number of years of useful 
                                   life of the capital item acquired, which
                                   useful life shall be determined reasonably by
                                   the owner of the North Garage in accordance
                                   with generally accepted accounting principles
                                   and practices in effect at the time of 
                                   acquisition of the capital item; and

                            (xi)   all other reasonable and necessary expenses
                                   paid in connection with the operating,
                                   cleaning and maintenance of the North Garage
                                   Improvement, the North Garage Site and said
                                   common areas and facilities and properly
                                   chargeable against income.

            If with respect to any full calendar month or fraction thereof
            falling within the Term after the Commencement Date of the 12CC
            Parking Lease, Monthly Operating Expenses Allocable to Each Parking
            Space for a full calendar month exceed Monthly Base Operating
            Expenses Allocable to Each Parking Space or for any such fraction of
            a calendar month exceed the corresponding fraction of Monthly Base
            Operating Expenses Allocable to Each Parking Space (such amount
            being hereinafter referred to as the "North Garage Operating Cost
            Excess"), then Tenant shall pay to Landlord, as Additional Rent, the
            product of (i) such North Garage Operating Cost Excess and (ii) the
            total number of automobiles for which Tenant has parking privileges
            in the North Garage under the terms of the Lease during such
            calendar month (the "North Garage Operating Cost Excess Payment").

            Payments by Tenant on account of the North Garage Operating Cost
            Excess Payment shall be made monthly at the time and in the fashion
            herein provided for the payment of Annual Fixed Rent. The amount so
            to be paid to Landlord shall be an amount from time to time
            reasonably estimated by Landlord to be sufficient to cover, in the
            aggregate, a sum equal to the North Garage Operating Cost Excess
            Payment for each calendar year during the Lease Term.




                                      -12-
<PAGE>   13

            No later than sixty (60) days after the end of each calendar year
            during the Lease Term or fraction thereof at the end of the Lease
            Term, Landlord shall render Tenant a statement in reasonable detail
            and according to usual accounting practices certified by a
            representative of Landlord, showing for the preceding calendar year
            or fraction thereof, as the case may be, the Operating Expenses for
            the North Garage for such calendar year or fraction thereof and the
            Operating Expenses Allocable to Each Parking Space for each calendar
            month during such calendar year or fraction thereof. Said statement
            to be rendered to Tenant also shall show for the preceding year or
            fraction thereof, as the case may be, the amounts already paid by
            Tenant on account of the North Garage Operating Cost Excess Payment
            and the amount of the North Garage Operating Cost Excess Payment
            remaining due from, or overpaid by, Tenant for the year or other
            period covered by the statement.

            If such statement shows a balance remaining due to Landlord, Tenant
            shall pay same to Landlord on or before the thirtieth (30th) day
            following receipt by Tenant of said statement. Any balance shown as
            due to Tenant shall be credited against Annual Fixed Rent next due,
            or refunded to Tenant if the Lease Term has then expired and Tenant
            has no further obligation to Landlord. Within sixty (60) days after
            receipt of such statement, time being of the essence, Tenant may
            notify ("Tenant's Operating Cost Notice") Landlord that Tenant
            reasonably believes that any of the costs included in such statement
            described in subparagraphs (i), (ii), (iv), (v), (vi) or (viii) of
            the definition of "Operating Expenses for the North Garage" are
            unreasonable when compared to the actual costs for the same items
            for other comparable garages in the City of Cambridge and Tenant
            shall, if available, include with Tenant's Operating Cost Notice
            reasonable evidence of such actual costs incurred by such other
            garage (the "Other Garage Costs"). If upon Landlord's receipt of
            Tenant's Operating Cost Notice and the Other Garage Costs (i) Tenant
            is not in default under the terms of this Lease, and (ii) Tenant
            shall have paid Landlord the full amount due under such statement,
            Landlord shall review the Other Garage Costs with Tenant. If upon
            such review, Landlord and Tenant agree that costs 




                                      -13-
<PAGE>   14
            reflected in such statement are unreasonable, Landlord shall take
            such reasonable actions as may be appropriate to reduce the costs so
            identified.

            (D) Notwithstanding the foregoing provisions of Sections 3 (A), 3
            (B) and 3 (C), in no event shall the monthly parking charge per
            vehicle set forth in Section 3 (A) (as escalated pursuant to
            Sections 3 (B) and 3 (C) for any month during the period from the
            Commencement Date of the 12CC Parking Lease to February 28, 2003
            exceed the monthly amount charged per space during such month by the
            operator of the North Garage (whether or not such operator is an
            affiliate of Landlord) to any single tenant leasing parking rights
            for fifty (50) or more vehicles in the North Garage.

      4.    There is added to the Lease a new Section 16.31 as follows:

            "16.31      TENANT'S OPTION TO PURCHASE DEMISED PREMISES. 

                        (A)     Upon and subject to the terms and conditions
                        contained in this Section and provided that (i) the
                        Lease (as herein amended) shall be in full force and
                        effect, (ii) there shall be no "Event of Default"
                        (defined in Section 15.1 of the Lease) either at the
                        time of the giving of the "Tenant's Option Exercise
                        Notice" (defined below) or on the Closing Date (as it
                        may be extended hereunder) and (iii) Tenant has neither
                        assigned the Lease nor sublet the Demised Premises
                        (except only as provided in Subsection (L) below)
                        Landlord hereby grants to Tenant the right and option to
                        purchase the Demised Premises. Landlord and Tenant
                        hereby agree that, subject to compliance with the terms
                        and conditions contained in this Section (including, but
                        not limited to Items (i), (ii) and (iii) set forth
                        immediately above), the within granted option to
                        purchase the Demised Premises shall remain superior to
                        the rights of any other person to purchase or otherwise
                        acquire the Demised Premises until February 28, 1998, it
                        being covenanted and agreed (a) that the within granted
                        option to purchase the Demised Premises shall not
                        prevent any sale, conveyance or other transfer of the
                        Demised Premises or any interest therein but any such
                        sale, conveyance or other 




                                      -14-
<PAGE>   15

                        transfer shall be subject to the within granted option
                        to purchase the Demised Premises upon and subject to the
                        terms and conditions hereof and (b) that the within
                        granted option to purchase the Demised Premises shall
                        not prevent any foreclosure, deed in lieu of foreclosure
                        or the exercise of any other rights under any mortgage
                        now or hereafter encumbering the Demised Premises but
                        that any person acquiring title to the Demised Premises
                        as a result of foreclosure, deed in lieu of foreclosure
                        or by the exercise of any such other rights shall be
                        subject to the within granted option to purchase the
                        Demised Premises upon and subject to the terms and
                        conditions hereof.

                        (B) (i) In order to exercise the within granted option
                        to purchase the Demised Premises, Tenant shall give
                        written notice to Landlord ("Tenant's Option Exercise
                        Notice") at any time on or before February 28, 1998
                        (time being of the essence). In order for Tenant's
                        Option Exercise Notice to be effective, it shall be
                        accompanied by, and Tenant shall pay together therewith,
                        a deposit in the amount of $1,347,240.00 in good funds
                        payable to Landlord (the "Deposit"); provided, however,
                        that the Deposit shall be promptly endorsed or otherwise
                        paid over to the "Escrow Agent" (defined in subsection
                        (B)(ii) hereof and shall be held and applied by the
                        Escrow Agent in accordance with the provisions of said
                        subsection (B)(ii) hereof. It is hereby covenanted and
                        agreed that if Tenant shall not give to Landlord
                        Tenant's Option Exercise Notice (together with the
                        Deposit) on or before February 28, 1998 (time being of
                        the essence), the within granted option to purchase the
                        Demised Premises shall automatically cease, expire and
                        be null and void without any action of the parties and
                        without any liability or obligation to or against any of
                        the parties. If Tenant shall



                                      -15-
<PAGE>   16

                        timely give to Landlord Tenant's Option Exercise Notice
                        (together with the Deposit), the Closing Date shall be
                        as set forth in subsection (D) hereof.

                        (B)     (ii) The "Escrow Agent" shall be the General
                        Counsel of Boston Properties, Inc., or such law firm,
                        title insurance company or other institutional escrow
                        agent as Landlord shall select. Landlord shall promptly
                        pay over the Deposit to the Escrow Agent so selected and
                        shall cause the Escrow Agent holding the Deposit to
                        acknowledge to Tenant receipt of the Deposit within a
                        reasonable period of time after the Escrow Agent
                        receives the Deposit. The Deposit shall be held in such
                        interest bearing account in such banking institution in
                        the City of Boston and upon such terms and conditions
                        relating to the deposit of funds and maintenance of
                        accounts as the Escrow Agent shall determine. The type
                        of account, the rate of interest, the terms and
                        conditions relating to the deposit of funds and
                        maintenance of accounts and the banking institution
                        shall be as solely selected by the Escrow Agent and the
                        Escrow Agent shall have no liability to Landlord or
                        Tenant respecting the selection of the type of account,
                        the rate of interest, the terms and conditions relating
                        to the deposit of funds and maintenance of accounts
                        and/or the banking institution. Further, the Deposit
                        shall be held by the Escrow Agent subject to the terms
                        of this Section 16.31 and shall be duly accounted for on
                        the "Closing Date" (hereinafter defined) as it may be
                        extended pursuant to this Section 16.31 or on the
                        earlier termination of this Section 16.31. All interest
                        earned on the Deposit shall be paid to Landlord with no
                        credit against the purchase price for such interest
                        being given to Tenant; provided, however, that if
                        pursuant to the terms of this Section 16.31 the Deposit
                        shall be returned to Tenant, the interest earned on the
                        Deposit at the time of such return of the Deposit shall
                        be paid over to Tenant.




                                      -16-
<PAGE>   17
                                If for any reason the closing does not occur and
                        either party makes a written demand upon the Escrow
                        Agent for delivery of the Deposit and the interest
                        earned thereon, the Escrow Agent shall give written
                        notice to the other party of such demand. If the Escrow
                        Agent does not receive a written objection from the
                        other party to the proposed payment within ten (10) days
                        after the giving of such notice, the Escrow Agent is
                        hereby authorized to make such delivery or payment. If
                        the Escrow Agent does receive such written objection
                        within such ten (10) business day period or if for any
                        other reason the Escrow Agent in good faith shall elect
                        not to make such payment, the Escrow Agent shall
                        continue to hold the Deposit until otherwise directed by
                        written instructions from both Landlord and Tenant or a
                        final judgment of a court. However, the Escrow Agent
                        shall have the right at any time to deposit the Deposit
                        with the clerk of such court of competent jurisdiction
                        in the Commonwealth of Massachusetts that the Escrow
                        Agent shall select. The Escrow Agent shall give written
                        notice of such deposit to Landlord and Tenant. Upon such
                        deposit the Escrow Agent shall be relieved and
                        discharged of all further obligations and
                        responsibilities hereunder.

                                The Landlord and Tenant acknowledge that the
                        Escrow Agent shall act solely as a stakeholder at
                        Landlord's and Tenant's request and for their
                        convenience, that the Escrow Agent shall not be deemed
                        to be the agent of either of the parties, and the Escrow
                        Agent shall not be liable to either of the parties for
                        any act or omission on its part unless taken or suffered
                        in bad faith, in willful disregard of this Section 16.31
                        or involving gross negligence. Landlord and Tenant shall
                        jointly and severally indemnify and hold the Escrow
                        Agent harmless from and against all costs,claims and
                        expenses, including reasonable attorneys' fees, 




                                      -17-
<PAGE>   18

                        incurred in connection with the performance of the
                        Escrow Agent's duties hereunder, except with respect to
                        actions or omissions taken or suffered by the Escrow
                        Agent in bad faith, in willful disregard of this Section
                        16.31 or involving gross negligence on the part of the
                        Escrow Agent. The parties agree that notwithstanding the
                        obligations of the Escrow Agent under this Agreement,
                        the Escrow Agent (if an attorney or law firm) shall be
                        permitted to represent Landlord or Tenant (as the case
                        may be) in connection with the transaction evidenced by
                        this Section 16.31 or in connection with any matters
                        arising from or related to this Section 16.31, the
                        consummation of this Section 16.31 or any claimed breach
                        of this Section 16.31 by any party.

                        (C)     (i) The purchase price payable for the Demised
                        Premises (the "Purchase Price") shall be THIRTEEN
                        MILLION FOUR HUNDRED SEVENTY TWO THOUSAND FOUR HUNDRED
                        DOLLARS ($13,472,400.00) payable on the closing by
                        Federal Funds immediately available to the Landlord at
                        The First National Bank of Boston, Boston, Massachusetts
                        or such other bank as may be stipulated by Landlord by
                        written notice to Tenant. Upon closing, credit shall be
                        given by Landlord to Tenant for and in the amount of the
                        Deposit (but not for the interest earned thereon). The
                        parties acknowledge and agree that "Impositions" (as
                        defined in Article VI of the Lease) are and shall
                        continue to be paid entirely by Tenant. Accordingly, no
                        credit or adjustment shall be given to Tenant at closing
                        (or otherwise) respecting Impositions.

                        (C)     (ii) Fixed Rent and all Additional Rent (except
                        respecting Impositions, provision for which is made in
                        Section 16.31 (C)(i) above) shall be paid through the
                        "Closing Date" (referred to in Section 16.31 (D) as it
                        may be extended pursuant to the provisions of this
                        Section 16.31 and in the event that




                                      -18-
<PAGE>   19
                        Tenant has prepaid amounts of the foregoing (excluding
                        Impositions) for periods extending beyond said Closing
                        Date (as it may be so extended), appropriate credit
                        shall be given to Tenant for amounts thereof (excluding
                        Impositions) prepaid for periods extending beyond said
                        Closing Date (as it may be so extended).

                        (D)     The closing (the "Closing") shall be held at the
                        Middlesex South District Registry of Deeds, 208
                        Cambridge Street, Cambridge, Massachusetts 02141 (or at
                        such other location in which said Registry of Deeds may
                        be located), or at such other place in the City of
                        Boston as the parties may agree, at 10:30 A.M. on that
                        date (the "Closing Date") which is thirty (30) days
                        after Landlord's receipt of Tenant's Option Exercise
                        Notice (accompanied by the Deposit); provided, however,
                        if said thirtieth (30th) day shall be a Saturday, Sunday
                        or Legal Holiday, the Closing Date shall be the next
                        following business day on which the Middlesex South
                        District Registry of Deeds shall be open for the
                        transaction of business. In addition to the provisions
                        of Section 16.31 (A) above, Landlord covenants that, so
                        long as this Section 16.31 shall be in full force and
                        effect and Tenant has not assigned this Lease nor sublet
                        the Premises (except only as provided in Subsection (L)
                        below) and Tenant shall not have wrongfully failed to
                        close on its purchase of the Demised Premises and there
                        shall be no "Event of Default" (defined in Section 15.1
                        (a) of the Lease (as herein amended), Landlord shall not
                        encumber the Demised Premises from time to time with any
                        mortgages (and other financing documents) in the
                        aggregate principal amount greater than the purchase
                        price. It is agreed that time is of the essence with
                        respect to the provisions and agreements in this Section
                        16.31.

                        (E)     The Demised Premises shall be sold and conveyed
                        in as-is condition and subject to




                                      -19-
<PAGE>   20
                        the following (collectively, the "Permitted
                        Encumbrances"):

                                (i)     the provisions of all laws, statutes,
                                        ordinances, rules and regulations
                                        (including, but not limited to, the
                                        Kendall Square Urban Renewal Plan (as
                                        amended and as it may hereafter be
                                        amended), building codes, zoning
                                        ordinances and regulations, and
                                        environmental laws and regulations) and
                                        the orders, rules, regulations and
                                        requirements of all Federal, state and
                                        municipal governments, and the
                                        appropriate agencies, officers,
                                        departments, boards and commissions
                                        thereof, whether now or hereafter in
                                        force, which may be applicable to the
                                        Demised Premises or the use or manner of
                                        use of the Demised Premises (herein
                                        collectively called "Legal
                                        Requirements");

                                (ii)    all leases, subleases and other
                                        occupancy agreements and arrangements
                                        for space in the Building of which
                                        Tenant has knowledge or which were
                                        entered into or arranged by Tenant (but
                                        nothing herein shall be construed as a
                                        waiver or modification of the provisions
                                        of Article XI of this Lease);

                                (iii)   any state of facts an accurate survey
                                        and/or inspection and/or review of any
                                        and all public records would disclose
                                        (including, without limitation, the
                                        condition of the Demised Premises and/or
                                        the presence , removal, containment,
                                        investigation, monitoring or permit
                                        conditions of or respecting hazardous
                                        materials in, on, under or with respect
                                        to the Demised Premises);



                                      -20-
<PAGE>   21

                                (iv)    real estate taxes and assessments, water
                                        and sewer charges, other municipal
                                        charges and assessments and all other
                                        Impositions for the fiscal tax year in
                                        which the closing shall occur and for
                                        all prior and subsequent fiscal tax
                                        years;

                                (v)     orders, agreements, decrees, license or
                                        permit conditions relating to, and
                                        notices of violation of, any Legal
                                        Requirements issued by any Federal,
                                        State or municipal or other governmental
                                        authorities, agencies, boards,
                                        departments or other instrumentalities
                                        having jurisdiction, against or
                                        affecting the Site, the Building or the
                                        use of the Site or Building including,
                                        without limitation, any of the foregoing
                                        arising out of or in any way related to
                                        the presence, removal, containment,
                                        investigation, monitoring or permit
                                        conditions of or respecting hazardous
                                        materials in, on, under or relating to
                                        the Demised Premises (herein
                                        collectively called "Governmental
                                        Directives");

                                (vi)    any lien or encumbrance placed on the
                                        Demised Premises (i) with the written
                                        consent of the Tenant (which consent
                                        shall not be unreasonably withheld or
                                        delayed), (ii) arising out of the use,
                                        occupancy or maintaining of the Site
                                        and/or the Building or any breach or
                                        default of the Tenant or (iii) resulting
                                        from any cause created by the act or
                                        omission of the Tenant;




                                      -21-
<PAGE>   22

                                (vii)   that certain unrecorded instrument
                                        entitled "Agreement For The Creation Of
                                        Certain Easements" dated September 19,
                                        1983 by and between COM/Energy Steam
                                        Company and Cambridge Center Associates
                                        and all easements, rights, grants and
                                        other agreements from time to time
                                        executed and/or granted pursuant to said
                                        Agreement;

                                (viii)  those matters set forth in Exhibit I
                                        attached hereto and hereby incorporated
                                        herein by reference;

                                (ix)    such other easements, agreements and
                                        restrictions of record insofar as in
                                        force (on the Closing Date as it may be
                                        extended hereunder) and applicable to
                                        the Site and/or Building, provided the
                                        same do not materially interfere with
                                        the use of the Demised Premises for the
                                        "Permitted Uses" (as defined in Section
                                        1.2 of the Lease).

                                (x)     The "Printed Exclusions" and the "Deemed
                                        Approved Matters" (both referred to in
                                        Section 16.31 I (2) hereof).

                (F) In the event of (i) a taking of the Demised Premises, or any
                part thereof, by the exercise of a right of condemnation or
                eminent domain, or (ii) damage to the Building by fire or other
                casualty, or (iii) a taking ,by the exercise of a right of
                condemnation or eminent domain, of the "Garage" (defined in the
                "Parking Garage Lease" hereinafter referred to at the end of
                this subsection) which taking pursuant to the provisions of
                Section 7 (b) of the Parking Garage Lease results in a
                termination of the Parking Garage Lease (a "Parking Garage
                Taking Termination"), and if any of the events referred to in
                items (i), (ii) and



                                      -22-
<PAGE>   23

                (iii) above occur between the date of Landlord's receipt of
                Tenant's Option Exercise Notice (given pursuant to and in
                compliance with the foregoing provisions hereof) and the
                Closing, then Tenant, by written notice to Landlord given within
                fifteen (15) days after such taking of the Demised Premises or
                any part thereof, a Parking Garage Taking Termination or the
                occurrence of such fire or casualty, as the case may be (but in
                no case given after the time of closing), shall elect one (1)
                but only one (1) of the following: (x) to cancel Tenant's
                exercise of the option to purchase the Demised Premises in which
                case the Deposit (and all interest then earned thereon) shall be
                promptly refunded to Tenant (provided the Deposit has previously
                been paid by Tenant) in which case the provisions of this
                Section 16.31 shall be void without further recourse or
                liability to or against either Landlord or Tenant provided,
                however, that the Lease (as amended by this Amendment but
                without this Section 16.31) shall remain in full force and
                effect in accordance with the terms of the Lease as amended by
                this Amendment but without this Section 16.31 or (y) to proceed
                to close regardless of the extent of such taking, damage or
                destruction (the "Closing Election").If Tenant shall make the
                Closing Election, the agreements contained in this Section 16.31
                shall remain unaffected thereby (except only as hereinafter
                specifically set forth in this Section 16.31 (F)) and the
                parties shall close the transaction as herein provided
                notwithstanding such occurrence, without any diminution or
                abatement of the purchase price; except, however, that unless
                Landlord has previously restored the Demised Premises to its
                former condition (Landlord having the right but not the
                obligation to do so), Landlord shall (i)(a) in the case of fire
                or casualty to the Building pay over or assign to Tenant, on
                delivery of the Deed, all amounts recovered or recoverable on
                account of an insured fire or casualty less any


                                      -23-
<PAGE>   24

                amounts reasonably expended by Landlord for any partial
                restoration or (i)(b) in the case of a taking of the Demised
                Premises (in whole or in part) under the power of eminent domain
                pay over or assign to Tenant, on delivery of the Deed, all
                amounts recovered on account of such taking and/or Landlord's
                claim in any such eminent domain or condemnation proceeding less
                any amounts reasonably expended by Landlord for any partial
                restoration, or (ii) if the holder of a mortgage on the Demised
                Premises shall require that fire or casualty proceeds or eminent
                domain (or condemnation) awards be applied on account of the
                mortgage indebtedness, give to Tenant a credit against the
                Purchase Price, on delivery of the Deed, equal to the insured
                fire or casualty proceeds received or recoverable or the eminent
                domain (or condemnation) awards respecting the Demised Premises,
                as the case may be, and retained by the holder of said mortgage
                less any amounts reasonably expended by Landlord for any partial
                restoration. In the event that Tenant shall fail to give any
                notice or to give timely notice pursuant to this Section 16.31
                (F) (time being of the essence), Tenant shall be deemed to have
                conclusively elected the Closing Election. The "Parking Garage
                Lease" is that certain Lease entitled "Cambridge Center North
                Garage Parking Lease" dated March 19, 1990 between David
                Barrett, Edward H. Linde and Mortimer B. Zuckerman, Trustees of
                Cambridge Center North Trust u/d/t dated August 7, 1988 recorded
                with the Middlesex South District Registry of Deeds in Book
                19383, Page 203, as landlord, and David Barrett, Edward H. Linde
                and Mortimer B. Zuckerman, Trustees of Fourteen Cambridge Center
                Trust u/d/t dated February 4, 1982 recorded with said Registry
                in Book 14707, Page 96, as tenant, a Notice of which is recorded
                with said Registry in Book 20450, Page 211.

                (G) The following deliveries shall be made at the Closing:




                                      -24-
<PAGE>   25

                        (i)     Landlord shall execute, acknowledge and deliver
                                a Massachusetts quitclaim deed to the Demised
                                Premises in recordable form, so as to convey to
                                Tenant good and clear record and marketable fee
                                simple title to the Demised Premises, free and
                                clear of all liens and encumbrances, except for,
                                and subject to, the Permitted Encumbrances.

                        (ii)    Each of Landlord and Tenant shall pay one half
                                (1/2) of all transfer taxes and stamp costs and
                                all other federal, state, county or municipal
                                taxes excises, impositions or levies applicable
                                to or imposed on the transfer of real property,
                                the conveyance of the Demised Premises or the
                                delivery or recording of the deed (whether now
                                or hereafter in effect) and whether assessed to
                                sellers or buyers of real property (excluding,
                                however, any income taxes of Landlord which
                                Landlord shall be obligated to pay and any
                                franchise, corporation or income taxes of Tenant
                                which Tenant shall be obligated to pay and
                                excluding all Impositions, provision for which
                                is made in Section 16.31 (C)(i) hereof) and
                                Tenant shall pay all recording costs by reason
                                of the delivery or recording of the deed.

                        (iii)   Landlord shall execute, acknowledge and deliver
                                to Tenant an assignment of all insurance
                                proceeds and condemnation awards or claims or
                                rights thereto, if any there be, then payable to
                                the Landlord, but as required by and subject to
                                the provisions of subsection (F) above, all
                                without representation or



                                      -25-
<PAGE>   26

                                warranty by or recourse against Landlord.

                        (iv)    The Tenant shall deliver the Purchase Price
                                described in subsection (C) above in the manner
                                specified therein.

                        (v)     The parties shall execute and deliver to each
                                other such other instruments and documents, and
                                shall pay or cause to be paid such sums of
                                money, to which either may be entitled pursuant
                                to any of the other provisions of this Section
                                or which may be required reasonably in
                                connection with the Closing and consistent with
                                the provisions of this Section. Each such
                                instrument and document to be delivered at the
                                Closing shall be consistent with the applicable
                                provisions of this Section, shall be in the form
                                or contain the information or provisions
                                provided for in this Section, and shall
                                otherwise be reasonably satisfactory in form and
                                substance to the parties.

                        (vi)    Landlord shall cause its counsel to deliver an
                                opinion in form and substance reasonably
                                satisfactory to Tenant relating to the power and
                                authority of Landlord to execute and deliver the
                                Deed and other instruments at the Closing.

                        (vii)   Tenant shall cause its counsel to deliver an
                                opinion in form and substance reasonably
                                satisfactory to Landlord relating to the power
                                and authority of Tenant to purchase the Demised
                                Premises and to execute and deliver any
                                instruments as are executed by the Tenant or its
                                nominee at the Closing.




                                      -26-
<PAGE>   27


                        (viii)  To the extent same are in Landlord's possession
                                or subject to Landlord's custody and control,
                                Landlord shall deliver to Tenant all licenses,
                                permits, authorizations and approvals of any
                                Governmental authorities relating to the Demised
                                Premises.

                (H) Tenant represents and warrants to the Landlord that: (i) it
                has examined, inspected and investigated, to its full
                satisfaction,the physical nature and condition of the Demised
                Premises; (ii) neither Landlord nor any agent, officer,
                director, employee, trustee, beneficiary, partner,
                representative or affiliate of Landlord (including, without
                limitation, Boston Properties, Inc., and its officers, directors
                and employees) has made any representation whatsoever regarding
                the Demised Premises or any part thereof, or anything relating
                to the subject matter of the agreements contained in this
                Section 16.31 including, without limiting the generality of the
                foregoing, representations as to the present or future physical
                nature or condition of the Demised Premises (including the
                presence, removal, containment, investigation, monitoring or
                permit conditions of or respecting hazardous materials),
                operation, size or zoning of the Demised Premises, operating
                expenses, carrying charges or real estate taxes and assessments,
                water and sewer charges, other municipal charges and assessments
                and all other Impositions affecting the Demised Premises; and
                (iii) it will take ownership of the Demised Premises in its "as
                is" condition on the Closing Date. The acceptance of a deed to
                the Demised Premises by Tenant shall be deemed to be a
                reaffirmation of the provisions of this Section 16.31 (H).

                (I) (1) (a) As of the Closing Date, Landlord shall be the owner
                of good and clear record and marketable fee simple title to the



                                      -27-
<PAGE>   28


                Demised Premises, subject only to those matters defined as
                Permitted Encumbrances in subsection (E) hereof. However, if on
                the Closing Date (i) the title to the Demised Premises is not as
                aforesaid or becomes additionally encumbered, in either case by
                an act or omission not attributable to Tenant, Tenant's
                affiliates or Tenant's subtenants, occupants, licensees, agents,
                contractors, subcontractors, tradesmen or materialmen or (ii)
                the Demised Premises do not comply in all material respects with
                applicable Legal Requirements or Governmental Directives or
                (iii) there is any material state of facts disclosed by an on
                the ground instrument survey of the Site performed and prepared
                for Tenant by the "Surveying Firm" (hereinafter defined) which
                renders title to the Site unmarketable (herein called "Survey
                Defects"), but in any of such cases not attributable to
                "Tenant's Operation Of The Demised Premises" (hereinafter
                defined), then Tenant, by written notice to Landlord given on or
                before the Closing Date (but in no event after the Closing Date)
                shall elect one (1) but only one (1) of the following: (x) to
                cancel Tenant's exercise of the option to purchase the Demised
                Premises in which case the Deposit (and all interest then earned
                thereon) shall be promptly refunded to Tenant (provided the
                Deposit has previously been paid by Tenant) and the provisions
                of this Section 16.31 shall be void and shall wholly cease and
                terminate and neither party shall have any claim against or
                liability to the other provided, however, that the Lease (as
                amended by this Amendment but excluding this Section 16.31)
                shall remain in full force and effect in accordance with the
                terms of the Lease as amended by this Amendment but without this
                Section 16.31 or (y) to extend the Closing Date for a period of
                ninety (90) days (the "Extension Election") in which case
                Landlord shall use reasonable efforts, but at Tenant's sole
                cost, expense and liability (collectively "Tenant's Cost And
                Liability") to attempt to bring the Demised Premises into


                                      -28-
<PAGE>   29

                material compliance with applicable Legal Requirements or
                Governmental Directives or to cure the Survey Defects, as the
                case may be, but in any of such cases Landlord shall have no
                obligation if any such non-compliance and/or Survey Defects is
                (are) the result of Tenant's Operation Of The Demised Premises.
                If Tenant shall make the Extension Election, the same shall only
                be effective if Tenant, in its notice to Landlord, shall agree
                to the provisions of item (y) hereof which shall survive the
                delivery of the deed and shall survive as elsewhere provided in
                this Section 16.31. For purposes hereof, "Tenant's Operation Of
                The Demised Premises" shall mean the manner and operation or use
                of the Demised Premises (or any portion or component thereof or
                equipment or process therein) by Tenant and/or those claiming
                by, through or under Tenant. For purposes hereof, the term
                "Surveying Firm" shall mean a duly licensed and qualified
                registered professional land surveying firm in the Commonwealth
                of Massachusetts first approved by Landlord, which approval
                shall not be unreasonably withheld or delayed. For purposes
                hereof, Landlord hereby approves the firm of Allen & Major
                presently located in Woburn, Massachusetts.

                (I) (1) (b) In addition to the foregoing provisions of Section
                16.31 (I)(1)(a), if on the Closing Date a hazardous materials
                analysis and study of the Site and its component materials
                performed by a geotechnical or other engineering first approved
                by Landlord (which approval shall not be unreasonably withheld
                or delayed) discloses the presence of hazardous materials in
                such quantities or amounts as to constitute a material violation
                or material non-compliance with the standards therefor set forth
                in applicable environmental laws, then Tenant, by written notice
                to Landlord given on or before the Closing Date (but in no event
                after the Closing Date) shall elect one (1) but only one (1) of
                the following: 



                                      -29-
<PAGE>   30
                (x) to cancel Tenant's exercise of the option to purchase the
                Demised Premises (the "Termination Election") in which case the
                Deposit (and all interest then earned thereon) shall be promptly
                refunded to Tenant (provided the Deposit has previously been
                paid by Tenant) and the provisions of this Section 16.31 shall
                be void and shall wholly cease and terminate and neither party
                shall have any claim against or liability to the other provided,
                however, that the Lease (as amended by this Amendment but
                excluding this Section 16.31) shall remain in full force and
                effect in accordance with the terms of the Lease as amended by
                this Amendment but without this Section 16.31 or (y) to close
                the purchase of the Demised Premises on the Closing Date without
                any deduction, offset or other reduction in the purchase price.
                In the case of an election to close under item (y), such
                election shall be conditioned upon Tenant executing a written
                instrument in favor of Landlord pursuant to which Tenant and its
                successors and assigns releases Landlord from liability
                respecting the presence, release, threat of release, removal and
                remediation of hazardous materials (herein called "Tenant's
                Release"). 

                (I) (1) (c) Subject to the provisions of Section 16.31 (I)
                (1)(a) above, to enable Landlord to make conveyance of the
                Demised Premises as herein provided, Landlord may, on the
                Closing Date (as it may be extended as above provided) use the
                purchase price or any portion thereof to clear the title to the
                Premises of any or all interests not permitted by this Section
                16.31 provided that all instruments so procured which affect
                title to the Demised Premises are recorded on the Closing Date
                (as it may be so extended) except, however, that Landlord shall
                have the right in lieu of payment and discharge to have
                deposited with Tenant's title insurer out of the purchase price
                such funds or assurances as will provide for the full payoff of
                all of such interests and the deletion of any exceptions for
                items other



                                      -30-
<PAGE>   31

                than the Permitted Exceptions, those matters deemed approved
                pursuant to Section 16.31 (I) (2) and the Printed Exclusions in
                the owner's policy of title insurance issued (or to be issued)
                to Tenant upon the acquisition of the Demised Premises by
                Tenant, in which case such interests shall not be considered
                objections to title to the Demised Premises.

                (I) (2) The Tenant agrees that it will obtain, not earlier than
                twenty (20) days prior to the date of Tenant's Option Exercise
                Notice nor later than ten (10) days prior to the Closing, a
                preliminary commitment from a recognized title insurance company
                reasonably selected by Tenant (the "Title Insurer"), covering
                the Demised Premises, pursuant to which the Title Insurer shall
                commit to issue to Tenant an Owner's Fee Title Insurance Policy
                insuring Tenant's title to the Demised Premises in the amount of
                the Purchase Price, subject only to the Permitted Encumbrances
                and the printed exclusions from coverage and other matters set
                forth in said preliminary commitment and the Owner's Policy of
                issue (the "Printed Exclusions"). A copy of title commitment
                shall be furnished to Landlord within seven (7) days after its
                receipt by the Tenant, accompanied by a written statement as to
                any objections to title set forth therein which are not
                Permitted Encumbrances or Printed Exclusions. Any objections to
                title then existing but not then raised in such written
                statement shall be deemed waived, accepted and approved by
                Tenant (the "Deemed Approved Matters").

                If Tenant shall have elected to extend the Closing Date as
                provided in and on the terms set forth in Section 16.31
                (I)(1)(a) and if, at the extended Closing Date, title to the
                Demised Premises shall not be as provided in this Section 16.31
                or if the Demised Premises are not in material compliance with
                the applicable Legal Requirements and Governmental Directives or
                if there shall be any Survey Defects, Tenant shall elect one




                                      -31-
<PAGE>   32

                (1) but only one (1) of the following: (a) to cancel its
                agreement to purchase the Demised Premises contained in this
                Section 16.31, in which event the Deposit (and all interest then
                earned thereon) shall be promptly refunded to Tenant (provided
                the Deposit shall have previously been paid by Tenant) and the
                provisions of this Section 16.31 shall be void and shall wholly
                cease and terminate, and neither party shall have any claim
                against or liability to the other provided, however, that the
                Lease as amended by this Amendment (but excluding the provisions
                of this Section 16.31) shall remain in full force and effect in
                accordance with the terms of this Lease as amended by this
                Amendment but without this Section 16.31; provided, however,
                that notwithstanding the foregoing Tenant's Cost And Liability
                shall survive, or (b) to consummate the Closing without any
                reduction of the purchase price or allowance against the same
                and without any liability on the part of the Landlord on account
                of the agreements or matters contained in this Section 16.31
                including, without limitation, any non-compliance with
                applicable Legal Requirements, any non-compliance with
                Governmental Directives, the existence of any Survey Defects,
                the existence of any Deemed Approved Matters and/or the
                existence or presence of any matters for which Tenant's Release
                is given.

                (J) If, on the Closing Date (as it may be extended pursuant to
                subsection (I) (1)(a) hereof), the Tenant shall fail to perform
                its obligation to purchase the Demised Premises, as herein
                provided, (i) the Landlord shall as its sole remedy therefor
                retain the Deposit (and all interest earned thereon) and (ii)
                the terms, conditions and provisions of both this Section 16.31
                and Section 16.32 shall wholly cease and terminate and neither
                party shall have any further claim against or liability to the
                other by reason of the provisions of this Section 16.31 and
                Section 16.32 (it being acknowledged that Landlord shall retain
                the Deposit and said interest); provided, however, that the
                Lease as amended by this Amendment (but excluding the provisions
                of this Section 16.31 and Section 



                                      -32-
<PAGE>   33

                16.32) shall remain in full force and effect in accordance with
                the terms of the Lease as amended by this Amendment but without
                this Section 16.31 and Section 16.32; provided, however, that
                notwithstanding the foregoing Tenant's Cost And Liability shall
                survive.

                (K) All notices, demands, requests, consents, approvals or other
                communications (for the purposes of this subsection collectively
                called "Notices") required or permitted to be given under this
                Section shall be in writing and shall be sent by registered or
                certified mail, return receipt requested, postage prepaid,
                addressed to the party to be notified at its address first above
                set forth or to such other address as such party shall have
                specified most recently by like Notice. At the same time any
                Notice is given to Landlord, a copy thereof shall be sent to
                Boston Properties, Inc., 8 Arlington Street, Boston,
                Massachusetts 02116, Attention: General Counsel. At the same
                time any Notice is given to Tenant a copy thereof shall be sent
                to Mintz, Levin, Cohn, Ferris, Glovsky And Popeo, P.C., One
                Financial Center, Boston, Massachusetts 02111, Attention: Joel
                R. Bloom, Esquire. Notices given as provided above shall be
                deemed given on the date of delivery except that if delivery is
                refused notice shall be deemed given on the date that delivery
                is first attempted to be made.

                (L) This Section 16.31, and Tenant's rights hereunder, shall not
                be assigned, pledged, hypothecated, mortgaged or otherwise
                transferred (collectively called "Transfer") and any purported
                Transfer shall be null and void and of no force or effect.
                However, Tenant shall have the right to assign this Section
                16.31 but only as part of an assignment of the entire Lease (as
                amended by this Amendment) to an 




                                      -33-
<PAGE>   34
                assignee permitted under Section 11.1 B of the Lease or
                consented to under Section 11.1 D of the Lease.

                (M) (1) With respect only to this Section 16.31, Landlord
                warrants and represents to Tenant that as of the date of this
                Amendment (a) Landlord is a nominee trust under the laws of the
                Commonwealth of Massachusetts and that it has the necessary
                power and authority under its Declaration of Trust to enter into
                this Fourth Amendment, (b) Landlord has no actual knowledge of
                any litigation pending against Landlord or the Demised Premises
                (excluding Tenant's Operation Of The Demised Premises as to
                which no representation or warranty is made) which would have a
                materially adverse effect on the obligations of Landlord under
                this Section 16.31, (c) the Lease (as amended by this Amendment)
                and those leases and subleases set forth in Exhibit I are the
                only leases entered into by Landlord respecting the Demised
                Premises (no warranty or representation being made as to any
                subleases or other occupancy agreements or arrangements made by
                Tenant or as to any matters set forth in Section 16.31 (E)(ii)
                hereof), (d) Landlord has not entered into any service contracts
                respecting the Demised Premises which will continue in effect
                beyond the conveyance of the Demised Premises to Tenant (or if
                any service contracts entered into by Landlord respecting the
                Demised Premises exist, the same shall be terminated on or
                before closing hereunder), (e) no consent, authorization or
                approval of any governmental body, authority or court is
                required in connection with Landlord's agreements set forth in
                this Section 16.31 (or if any of the same shall be required,
                Landlord has obtained or will obtain same) and (f) Landlord has
                not entered into any options to sell, nor granted rights of
                first offer or first refusal to sell nor entered into any other
                written agreements to sell the 




                                      -34-
<PAGE>   35

                Demised Premises which would deprive Tenant of its option to
                purchase the Demised Premises under this Section 16.31;
                provided, however, Tenant acknowledges that Landlord's mortgage
                lender must approve and consent to this Amendment and the
                existence of any mortgages does not contravene the foregoing. At
                the time of closing, the then owner of the Demised Premises
                shall reaffirm the foregoing warranties and representations or
                state in what respects the same are not then true and correct.
                The provisions hereof shall not survive the delivery of the deed
                of the Demised Premises.

                (M)(M) (2) If any of the warranties and representations set
                forth in subsection (M) (1) above shall not be complied with in
                material respects as of the Closing Date (as it may be extended
                pursuant to the applicable provisions hereof), Tenant shall
                elect one (1) but only one (1) of the following: (a) to cancel
                its agreement to purchase the Demised Premises contained in this
                Section 16.31 in which event the Deposit (and all interest then
                earned thereon) shall be promptly refunded to Tenant (provided
                the Deposit shall have been paid by Tenant) and the provisions
                of this Section 16.31 shall be void and shall wholly cease and
                terminate, and neither party shall have any claim against or
                liability to the other provided, however, that the Lease as
                amended by this Amendment (but excluding the provisions of this
                Section 16.31) shall remain in full force and effect in
                accordance with the terms of this Lease as amended by this
                Amendment but without this Section 16.31, or (b) to consummate
                the Closing without any reduction of the purchase price or
                allowance against same and without any liability on the part of
                Landlord on account of the matters set forth in subsection (M)
                (1) above.

                (N) It is understood and agreed that all understandings and
                agreements heretofore had between the parties hereto with
                respect to



                                      -35-
<PAGE>   36
                the subject matter of this Section 16.31 are merged herewith,
                and this Section 16.31 alone fully and completely expresses
                their agreement.

                (O) The delivery and acceptance of the Deed conveying fee title
                to the Demised Premises shall be deemed to be an
                acknowledgement, for all purposes, of the full performance and
                discharge of every representation, warranty, agreement and
                obligation on the part of each of the parties to be performed
                pursuant to the provisions of this Section 16.31, except those
                which are herein specifically stated to survive the Closing and
                the delivery of the Deed.

                (P) If the Lease is terminated on account of an Event of Default
                specified in Section 15.1 of the Lease, the provisions contained
                in this Section 16.31 shall terminate and shall not survive the
                termination of the Lease.

                (Q) The Tenant acknowledges that a reference to the Landlord
                herein is a reference to the Trustees of Fourteen Cambridge
                Center Trust under Declaration of Trust identified on the first
                page of this Amendment, and that no trustee, nor any beneficiary
                of said trust, nor any officer, director, employee or agent of
                said trust or any affiliate of said trust (including, but not
                limited to, Boston Properties, Inc., Mortimer B. Zuckerman,
                Edward H.Linde and/or any affiliates of said Mortimer B.
                Zuckerman and/or Edward H. Linde) nor any successor holder of
                Landlord's interest in the Lease (as amended) and/or in the
                Demised Premises, shall be held to any personal liability
                hereunder, nor shall resort be had to their private property for
                the satisfaction of any claim hereunder, and Tenant agrees to
                look solely to the Demised Premises in satisfaction of any
                liability of the Landlord or any such successor under this
                Section 16.31. In no event shall any of the aforesaid persons or
                parties (including, without limitation, Landlord and its




                                      -36-
<PAGE>   37

                successors) be liable for indirect or consequential damages.

                (R) The unenforceability of invalidity of any one or more
                provisions hereof shall not affect the validity or
                enforceability of any of the other provisions hereof.

                (S) This Section shall be governed by, and construed and
                enforced in accordance with the law of, The Commonwealth of
                Massachusetts, as the same may from time to time exist.

                (T) The Lease (as herein amended) shall terminate upon and as of
                the date of the acquisition by Tenant of title to the Demised
                Premises pursuant to this Section 16.31".

      5.    There is added to the Lease a new Section 16.32 as set forth in
            Exhibit II attached hereto and hereby incorporated herein by
            reference.

      6.    The existing Section 16.32 of the Lease is hereby renumbered as
            Section 16.33.

      7.    For purposes of Section 11.1 D(a) of the Lease, the proposed
            assignee shall be deemed to possess "adequate financial capability
            to meet the tenant obligations" under the Lease (as herein amended)
            if the proposed assignee (a) has a shareholder's or owner's (as
            applicable) equity as determined in accordance with "GAAP"
            (hereinafter defined) at least equal to One Hundred Twenty Five
            Million Dollars ($125,000,000.00) plus the "CPI Amount" (hereinafter
            defined), (b) has Net Income (as determined in accordance with
            GAAP), but excluding interest and investment income and income from
            extraordinary events, of at least (1) Seven Million Dollars
            ($7,000,000.00) plus the CPI Amount for the fiscal year immediately
            prior to such proposed assignment, (2) One Dollar ($1.00) for each
            of at least two of the last three fiscal years prior to such
            proposed assignment and (3) Twenty Million Dollars ($20,000,000.00)
            plus the CPI Amount in the aggregate for the last three fiscal years
            prior to such proposed assignment and (c) has Fifty Million Dollars
            ($50,000,000.00) plus the CPI Amount in cash or marketable
            securities at the time of such proposed




                                      -37-
<PAGE>   38

            assignment. For purposes hereof, "GAAP" shall mean generally
            accepted accounting principles consistently applied throughout
            (and/or with respect to) the relevant period. In addition and for
            purposes hereof, the "CPI Amount" shall be and mean an amount equal
            to the product of the amount in question times the percentage
            increase, if any, in the Consumer Price Index (1982-1984 = 100) of
            all items for urban wage earners and clerical workers published by
            the Bureau of Labor Statistics of the U.S. Department of Labor for
            Boston, Massachusetts (the "Index") (or if there ceases to be any
            such publication, any other substantially equivalent index generally
            recognized to measure changes in the cost of living for Boston,
            Massachusetts) between the Index last published prior to the date of
            this Amendment and the Index last published prior to the change
            contemplated by this Section.

      8.    Reference is made to the "East Garage Sublease" (referred to in Item
            5 of Exhibit I attached hereto). Notwithstanding anything contained
            in the Lease as amended by this Amendment, Tenant acknowledges,
            covenants and agrees that Landlord shall have the right, in its sole
            and absolute discretion, to terminate the East Garage Sublease or to
            cause the East Garage Sublease to be terminated at such time
            (whether during the Term of the Lease as amended hereby (as it may
            be extended) or after any conveyance of the Demised Premises to
            Tenant pursuant to Section 16.31 or Section 16.32 or otherwise) as
            Landlord, in its sole and absolute discretion, shall determine.

      9.    Landlord and Tenant each represents and warrants to the other that
            it has not dealt with any real estate brokers or other persons or
            entities which have been, are or will be entitled to any broker's or
            finder's fee or any similar commission or fee in connection with
            this Lease Amendment (including, without limitation, the
            transactions contemplated by Sections 16.31 and 16.32 of the Lease
            and added to the Lease pursuant to this Lease Amendment) except
            Fallon, Hines & O'Connor (the "Recognized Broker"). Landlord and
            Tenant each agree to indemnify, hold harmless, protect and defend
            the other from and against any and all loss, damage, liability and
            expense, including costs and reasonable attorneys' fees which such
            other party incurs or




                                      -38-
<PAGE>   39
            sustains by reason of the breach by the indemnifying party of its
            foregoing warranties and representations. Tenant covenants and
            agrees that it shall be solely responsible for and shall pay to the
            Recognized Broker such fee or commission as shall be due to the
            Recognized Broker. Tenant shall defend, hold harmless and indemnify
            Landlord (and its affiliates including, without limitation, Boston
            Properties, Inc.) from and against any claims by the Recognized
            Broker. The provisions hereof shall survive the expiration or any
            termination of this Lease (as herein amended), the termination of
            this Fourth Amendment To Lease pursuant to the provisions of Section
            11 hereof, the expiration or any termination of Section 16.31 and/or
            Section 16.32 hereof and/or the delivery of any deed of the Demised
            Premises to Tenant pursuant to Section 16.31 hereof, Section 16.32
            hereof or otherwise.

      10.   Concurrently with the execution of this Fourth Amendment to Lease,
            Landlord shall deliver to Tenant a Trustees' Certificate respecting
            the authority to enter into this Fourth Amendment To Lease and
            Tenant shall deliver to Landlord a corporate vote evidencing the
            authority of Tenant to enter into this Fourth Amendment to Lease.

      11.   Reference is made to that certain lease of even date herewith
            between North Parcel Limited Partnership, a Massachusetts Limited
            Partnership ("NPLP"), as landlord, and Biogen Realty Limited
            Partnership, a Massachusetts Limited Partnership ("BRLP") that is an
            affiliate of Tenant, as tenant (the "Tract V Lease"), pursuant to
            which Tract V Lease NPLP leased to BRLP that certain parcel of
            unimproved land therein referred to as Tract V. BRLP is to construct
            the "Improvements" (as therein defined) on said Tract V in
            accordance with the requirements of the Tract V Lease and the "Land
            Disposition Agreement" (hereinafter defined). The Land Disposition
            Agreement is that certain Supplemental Land Disposition Agreement
            dated October 6, 1993 between the Cambridge Redevelopment Authority
            (the "Authority") and NPLP. The parties acknowledge that the
            transaction contemplated by the Tract V Lease was and is a material
            inducement to the parties to enter into this Fourth Amendment To
            Lease, and that their agreement herein is dependent on the
            successful completion of said Tract V transaction. Therefore,
            notwithstanding anything to



                                      -39-
<PAGE>   40

            the contrary set forth in this Fourth Amendment To Lease, (i) this
            Fourth Amendment To Lease shall automatically terminate, cease and
            expire if construction of the Improvements on Tract V is not
            commenced on or before October 1, 1994 or (ii) if construction of
            the Improvements on Tract V is so commenced on or before said
            October 1, 1994, this Fourth Amendment To Lease shall automatically
            terminate, cease and expire if construction of the Improvements is
            not completed and a Certificate of Completion is not issued by the
            Authority for the Improvements on or before the date set forth for
            completion of the Improvements on Tract V pursuant to the Land
            Disposition Agreement. In the event of any such termination of this
            Fourth Amendment To Lease, the Lease (excepting this Fourth
            Amendment To Lease) shall remain unchanged and in full force and
            effect in accordance with its terms (excepting this Fourth Amendment
            To Lease).

      12.   All capitalized terms and words used in this Amendment shall have
            the same meaning as set forth in the Lease unless a contrary meaning
            is expressly set forth herein.

      13.   Except as expressly amended hereby, the Lease and its terms and
            provisions shall remain unchanged and in full force and effect.




                                      -40-
<PAGE>   41


      EXECUTED under seal as of the date and year first above written.


WITNESS:                                    LANDLORD:

--------------------------------         -------------------------------------

                                         EDWARD H. LINDE, TRUSTEE OF
                                         FOURTEEN CAMBRIDGE CENTER TRUST AND 
                                         NOT INDIVIDUALLY



                                         TENANT:

                                         BIOGEN, INC.


                                         By:
                                            ---------------------------------- 
                                            Name: James C. Mullen
                                            Title: PRESIDENT
                                                   (VICE PRESIDENT)

ATTEST:

By:                                   By: 
   ------------------------------         ------------------------------------
   Name: Michael J. Astrue                Name: Timothy M. Kish
   Title: ASSISTANT CLERK                 Title: TREASURER
                                                 (ASSISTANT TREASURER)