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                                                                   EXHIBIT 10.25

                         STANDARD FORM COMMERCIAL LEASE

1.   PARTIES                          68 Moulton Street Realty Trust, a
     (FILL IN)                        Massachusetts Trust with its mailing
                                      address of P.O. Box 95, Winchester, MA
                                      01890

                                      LESSOR, which expression shall include
                                      their              heirs, successors, and
                                      assigns where the context so admits, does
                                      hereby lease to Momenta Pharmaceuticals,
                                      Inc., a Massachusetts corporation with its
                                      principal place of business at 43 Moulton
                                      Street, Cambridge, MA 02138.

                                      LESSEE, which expression shall include its
                                      successors, executors, administrators, and
                                      assigns where the context so admits, and
                                      the LESSEE hereby leases the following
                                      described premises:

2.   PREMISES                         4,316 square feet of office space located
     (FILL IN AND INCLUDE, IF         on the third floor of 68 Moulton Street,
     APPLICABLE, SUITE NUMBER,        Cambridge, MA 02138.
     FLOOR NUMBER, AND
     SQUARE FEET)                     together with the right to use in common,
                                      with others entitled thereto, the
                                      hallways, stairways, and elevators,
                                      necessary for access to sold leased
                                      premises, and lavatories nearest thereto.

3.   TERM                             The term of this lease shall be for eight
     (FILL IN)                        (8) months commencing on November 1, 2003
                                      and ending on June 30, 2004.

4.   RENT                             The LESSEE shall pay to the LESSOR fixed
     (FILL IN)                        rent at the rate of $61,862.64, for a 8
                                      month period, payable in advance in
                                      monthly installments of $7,732.83 subject
                                      to proration in the case of any partial
                                      calendar month. All rent shall be payable
                                      without offset or deduction. Rent is
                                      calculated on the rate of $20.00 per
                                      square foot with an additional charge of
                                      $1.50 per square foot for electricity.

5.   SECURITY                         Upon the execution of this lease, the
     DEPOSIT                          LESSEE shall pay to the LESSOR the amount
     (FILL IN)                        of $15,465.66 dollars, which shall be held
                                      as a security for the LESSEE's performance
                                      as herein provided and refunded to the
                                      LESSEE at the end of this lease, without
                                      interest, subject to the LESSEE's
                                      satisfactory compliance with the
                                      conditions hereof.

6.   RENT
     ADJUSTMENT

     A.  TAX
         ESCALATION
         (fill in or delete)

     B.  OPERATING
         COST
         ESCALATION
         (FILL IN OR DELETE)

     C.  CONSUMER
         PRICE
         ESCALATION
         (FILL IN OR DELETE)

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7.   UTILITIES                        The LESSOR agrees to provide all utility
                                      service and to furnish reasonably hot and
                                      cold water and reasonable heat and air
                                      conditioning to the leased premises, the
                                      hallways, stairways, elevators, and
                                      lavatories during normal business hours on
                                      regular business days of the heating and
                                      air conditioning seasons of each year, to
                                      furnish elevator service and to light
     *DELETE AIR CONDITIONING"        passageways and stairways during business
     IF NOT APPLICABLE                hours, and to furnish such cleaning
                                      service as is customary in similar
                                      buildings in said city or town, all
                                      subject to interruption due to any
                                      accident, to the making of repairs,
                                      alterations, or improvements, to labor
                                      difficulties, to trouble in obtaining
                                      fuel, electricity, service, or supplies
                                      from the sources from which they are
                                      usually obtained for said building, or to
                                      any cause beyond the LESSOR's control.

                                      LESSOR shall have no obligation to provide
                                      utilities or equipment other than the
                                      utilities and equipment within the
                                      premises as of the commencement date of
                                      this lease. In the event LESSEE requires
                                      additional utilities or equipment, the
                                      installation and maintenance thereof shall
                                      be the LESSEE's sole obligation, provided
                                      that such installation shall be subject to
                                      the written consent of the LESSOR.

8.   USE OF LEASED                    The LESSEE shall use the leased premises
     PREMISES                         only for the purpose of General office
     (FILL IN)                        activities.

9.   COMPLIANCE WITH                  The LESSEE acknowledges that no trade or
     LAWS                             occupation shall be conducted in the
                                      leased premises or use made thereof which
                                      will be unlawful, improper, noisy or
                                      offensive, or contrary to any law or any
                                      municipal by-law or ordinance in force in
                                      the city or town in which, the premises
                                      are situated. Without limiting the
                                      generality of the foregoing (a) the LESSEE
                                      shall not bring or permit to be brought or
                                      kept in or on the leased premises or
                                      elsewhere on the LESSOR's property any
                                      hazardous, toxic, inflammable, combustible
                                      or explosive fluid, material, chemical or
                                      substance, including without limitation
                                      any item defined as hazardous pursuant to
                                      Chapter 21E of the Massachusetts General
                                      Laws; and (b) the LESSEE shall be
                                      responsible for compliance with
                                      requirements imposed by the Americans with
                                      Disabilities Act relative to the layout of
                                      the leased premises and any work performed
                                      by the LESSEE therein.

10.  FIRE INSURANCE                   The LESSEE shall not permit any use of the
                                      leased premises which will make voidable
                                      any insurance on the property of which the
                                      leased premises are a part, or on the
                                      contents of said property or which shall
                                      be contrary to any law or regulation from
                                      time to time established by the New
                                      England Fire Insurance Rating Association,
                                      or any similar body succeeding to its
                                      powers. The LESSEE shall on demand
                                      reimburse the LESSOR, and all other
                                      tenants, all extra insurance premiums
                                      caused by the LESSEE's use of the
                                      premises.

11.  MAINTENANCE                      The LESSEE agrees to maintain the leased
                                      premises in good condition, damage by fire
     A.  LESSEE'S                     and other casualty only excepted, and
         OBLIGATIONS                  whenever necessary, to replace plate glass
                                      and other glass therein, acknowledging
                                      that the leased premises are now in good
                                      order and the glass whole. The LESSEE
                                      shall not permit the leased premises to be
                                      overloaded, damaged, stripped, or defaced,
                                      nor suffer any waste. LESSEE shall obtain
                                      written consent of LESSOR before erecting
                                      any sign on the premises.

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     B.  LESSOR'S                     The LESSOR agrees to maintain the
         OBLIGATIONS                  structure of the building of which the
                                      leased premises are a part in the same
                                      condition as it is at the commencement of
                                      the term or as it may be put in during the
                                      term of this lease, reasonable wear and
                                      tear, damage by fire and other casualty
                                      only excepted, unless such maintenance is
                                      required because of the LESSEE or those
                                      for whose conduct the LESSEE is legally
                                      responsible.

12.  ALTERATIONS --                   The LESSEE shall not make structural
     ADDITIONS                        alterations or additions to the leased
                                      premises, but may make non-structural
                                      alterations provided the LESSOR consents
                                      thereto in writing, which consent shall
                                      not be unreasonably withheld or delayed.
                                      All such allowed alterations shall be at
                                      LESSEE's expense and shall be in quality
                                      at least equal to the present
                                      construction. LESSEE shall not permit any
                                      mechanics' liens, or similar liens, to
                                      remain upon the leased premises for labor
                                      and material furnished to LESSEE or
                                      claimed to have been furnished to LESSEE
                                      in connection with work of any character
                                      performed or claimed to have been
                                      performed at the direction of LESSEE and
                                      shall cause any such lien to be released
                                      of record forthwith without cost to
                                      LESSOR. Any alterations or improvements
                                      made by the LESSEE shall become the
                                      property of the LESSOR at the termination
                                      of occupancy as provided herein.

13.  ASSIGNMENT --                    The LESSEE shall not assign or sublet the
     SUBLEASING                       whole or any part of the leased premises
                                      without LESSOR's prior written consent.
                                      Notwithstanding such consent, LESSEE shall
                                      remain liable to LESSOR for the payment of
                                      all rent and for the full performance of
                                      the covenants and conditions of this
                                      lease.

14.  SUBORDINATION                    This lease shall be subject and
                                      subordinate to any and all mortgages,
                                      deeds of trust and other instruments in
                                      the nature of a mortgage, now or at any
                                      time hereafter, a lien or liens on the
                                      property of which the leased premises are
                                      a part and the LESSEE shall, when
                                      requested, promptly execute and deliver
                                      such written instruments as shall be
                                      necessary to show the subordination of
                                      this lease to said mortgages, deeds of
                                      trust or other such instruments in the
                                      nature of a mortgage.

15.  LESSOR'S ACCESS                  The LESSOR or agents of the LESSOR may, at
                                      reasonable times, enter to view the leased
                                      premises and may remove placards and signs
                                      not approved and affixed as herein
                                      provided, and make repairs and alterations
                                      as LESSOR should elect to do and may show
                                      the leased premises to others, and at any
                                      time affix to any suitable part of the
                                      leased premises a notice for letting or
                                      selling the leased premises or property of
                                      which the leased premises are a part and
                                      keep the same so affixed without hindrance
                                      or molestation.

16.  INDEMNIFICATION                  The LESSEE shall save the LESSOR harmless
     AND LIABILITY                    from all loss and damage occasioned by
     (FILL IN)                        anything occurring on the leased premises
                                      unless caused by the negligence or
                                      misconduct of the LESSOR, and from all
                                      loss and damage wherever occurring
                                      occasioned by any omission, fault, neglect
                                      or other misconduct of the LESSEE. The
                                      removal of snow and ice from the sidewalks
                                      bordering upon the leased premises shall
                                      be Lessor's responsibility.

17.  LESSEE'S LIABILITY               The LESSEE shall maintain with respect to
     INSURANCE                        the leased premises and the property of
     (FILL IN)                        which the leased premises are a part
                                      comprehensive public liability insurance
                                      in the amount of $2,000,000.00 with
                                      property damage insurance in limits of
                                      $2,000,000.00 in responsible companies
                                      qualified to do business in Massachusetts
                                      and in good standing therein insuring the
                                      LESSOR as well as LESSEE against injury to
                                      persons or damage to properly as provided.
                                      The LESSEE shall deposit with the LESSOR
                                      certificates for such insurance at or
                                      prior to the commencement of the term, and
                                      thereafter within thirty (30) days prior
                                      to the expiration of any such policies.
                                      All such insurance certificates shall
                                      provide that such policies shall not be

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                                      cancelled without at least ten (10) days
                                      prior written notice to each assured named
                                      therein.

18.  FIRE, CASUALTY --                Should a substantial portion of the leased
     EMINENT DOMAIN                   premises, or of the property of which they
                                      are a part, be substantially damaged by
                                      fire or other casualty, or be taken by
                                      eminent domain, the LESSOR may elect to
                                      terminate this lease. When such fire,
                                      casualty, or taking renders the leased
                                      premises substantially unsuitable for
                                      their intended use, a just and
                                      proportionate abatement of rent shall be
                                      made, and the LESSEE may elect to
                                      terminate this lease if:

                                        (a)   The LESSOR fails to give written
                                              notice within thirty (30) days of
                                              intention to restore leased
                                              premises, or

                                        (b)   The LESSOR falls to restore the
                                              leased premises to a condition
                                              substantially suitable for their
                                              intended use within ninety (90)
                                              days of said fire, casualty or
                                              taking.

                                      The LESSOR reserves, and the LESSEE grants
                                      to the LESSOR, all rights which the LESSEE
                                      may have for damages or injury to the
                                      leased premises for any taking by eminent
                                      domain, except for damage to the LESSEE's
                                      fixtures, property, or equipment.

19.  DEFAULT AND                      In the event that:
     BANKRUPTCY
     (FILL IN)                          (a)   The LESSEE shall default in the
                                              payment of any installment of rent
                                              or other sum herein specified and
                                              such default shall continue for
                                              ten (10) days after written notice
                                              thereof; or

                                        (b)   The LESSEE shall default in the
                                              observance or performance of any
                                              other of the LESSEE's covenants,
                                              agreements, or obligations
                                              hereunder and such default shall
                                              not be corrected within thirty
                                              (30) days after written notice
                                              thereof; or

                                        (c)   The LESSEE shall be declared
                                              bankrupt or insolvent according to
                                              law, or, if any assignment shall
                                              be made of LESSEE's property for
                                              the benefit of creditors,

                                      then the LESSOR shall have the right
                                      thereafter, while such default continues,
                                      to re-enter and take complete possession
                                      of the leased premises, to declare the
                                      term of this lease ended, and remove the
                                      LESSEE's effects, without prejudice to any
                                      remedies which might be otherwise used for
                                      arrears of rent or other default. The
                                      LESSEE shall indemnify the LESSOR against
                                      all loss of rent and other payments which
                                      the LESSOR may incur by reason of such
                                      termination during the residue of the
                                      term. If the LESSEE shall default, after
                                      reasonable notice thereof, in the
                                      observance or performance of any
                                      conditions or covenants on LESSEE's part
                                      to be observed or performed under or by
                                      virtue of any of the provisions in any
                                      article of this lease, the LESSOR, without
                                      being under any obligation to do so and
                                      without thereby waiving such default, may
                                      remedy such default for the account and at
                                      the expense of the LESSEE. If the LESSOR
                                      makes any expenditures or incurs any
                                      obligations for the payment of money in
                                      connection therewith, including but not
                                      limited to, reasonable attorney's fees in
                                      instituting, prosecuting or defending any
                                      action or proceeding, such sums paid or
                                      obligations insured, with interest at the
                                      rate of 18% per cent per annum and costs,
                                      shall be paid to the LESSOR by the LESSEE
                                      as additional rent.

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20.  NOTICE                           Any notice from the LESSOR to the LESSEE
     (FILL IN)                        relating to the leased premises or to the
                                      occupancy thereof, shall be deemed duly
                                      served, if left at the leased premises
                                      addressed to the LESSEE, or if mailed to
                                      the leased premises, registered or
                                      certified mail, return receipt requested,
                                      postage prepaid, addressed to the LESSEE.
                                      Any notice from the LESSEE to the LESSOR
                                      relating to the leased premises or to the
                                      occupancy thereof, shall be deemed duly
                                      served, if mailed to the LESSOR by
                                      registered or certified mail, return
                                      receipt requested, postage prepaid,
                                      addressed to the LESSOR at such address as
                                      the LESSOR may from time to time advise in
                                      writing. All rent notices shall be paid
                                      and sent to the LESSOR at P.O. Box 95,
                                      Winchester, MA 01890

21. SURRENDER                         The LESSEE shall at the expiration or
                                      other termination of this lease remove all
                                      LESSEE's goods and effects from the leased
                                      premises, (including. without hereby
                                      limiting the generality of the foregoing,
                                      all signs and lettering affixed or painted
                                      by the LESSEE, either inside or outside
                                      the leased premises). LESSEE shall deliver
                                      to the LESSOR the leased premises and all
                                      keys, locks thereto and other fixtures
                                      connected therewith and all alterations
                                      and additions made to or upon the leased
                                      premises, in good condition, damage by
                                      fire or other casualty only excepted. In
                                      the event of the LESSEE's failure to
                                      remove any of LESSEE's property from the
                                      premises, LESSOR is hereby authorized,
                                      without liability to LESSEE for loss or
                                      damage thereto, and at the sole risk of
                                      LESSEE, to remove and store any of the
                                      property at LESSEE's expense, or to retain
                                      same under LESSOR's control or to sell at
                                      public or private sale, without notice any
                                      or all of the property not so removed and
                                      to apply the net proceeds of such sale to
                                      the payment of any sum due hereunder, or
                                      to destroy such property.

22.  BROKERAGE                        The Broker(s) named herein , Spaulding &
     (FILL IN OR DELETE)              Slye Co., Inc. warrant(s) that he (they)
                                      is (are) duly licensed as such by the
                                      Commonwealth of Massachusetts, and join(s)
                                      in this agreement and become(s) a party
                                      hereto, insofar as any provisions of this
                                      agreement expressly apply to him (them),
                                      and to any amendments or modifications of
                                      such provisions to which he (they)
                                      agree(s) in writing.

                                      LESSOR agrees to pay the above-named
                                      Broker upon the term commencement date a
                                      fee for professional services of Two
                                      Thousand, Eight Hundred Seventy-Seven and
                                      33/100 ($2,877.33) Dollars. The LESSEE
                                      warrants and represents that it has dealt
                                      with no other broker entitled to claim a
                                      commission in connection with this
                                      transaction and shall indemnify the LESSOR
                                      from and against any such claim, including
                                      without limitation reasonable attorneys'
                                      fees incurred by the LESSOR in connection
                                      therewith.

23.  CONDITION OF                     Except as may be otherwise expressly set
     PREMISES                         forth herein, the LESSEE shall accept the
                                      leased premises "as is" in their condition
                                      as of the commencement of the term of this
                                      lease, and the LESSOR shall be obligated
                                      to perform no work whatsoever in order to
                                      prepare the leased premises for occupancy
                                      by the LESSEE.

24.  FORCE MAJEURE                    In the event that the LESSOR is prevented
                                      or delayed from making any repairs or
                                      performing any other covenant hereunder by
                                      reason of any cause reasonably beyond the
                                      control of the LESSOR, the LESSOR shall
                                      not be liable to the LESSEE therefor nor,
                                      except as expressly otherwise provided in
                                      case of casualty or taking, shall the
                                      LESSEE be entitled to any abatement or
                                      reduction of rent by reason thereof, nor
                                      shall the same give rise to a claim by the
                                      LESSEE that such failure constitutes
                                      actual or constructive eviction from the
                                      leased premises or any part thereof.

25.  LATE CHARGE                      If rent or any other sum payable hereunder
                                      remains outstanding for a period of ten
                                      (10) days, the LESSEE shall pay to the
                                      LESSOR a late charge equal to one and

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                                      one-half percent (1.5%) of the amount due
                                      for each month or portion thereof during
                                      which the arrearage continues.

26.  LIABILITY OF                     No owner of the property of which the
     OWNER                            leased premises are a part shall be liable
                                      hereunder except for breaches of the
                                      LESSOR's obligations occurring during the
                                      period of such ownership. The obligations
                                      of the LESSOR shall be binding upon the
                                      LESSOR's interest in said property, but
                                      not upon other assets of the LESSOR, and
                                      no individual partner, agent, trustee,
                                      stockholder, officer, director, employee
                                      or beneficiary of the LESSOR shall be
                                      personally liable for performance of the
                                      LESSOR's obligations hereunder.

27.  OTHER PROVISIONS

IN WITNESS WHEREOF, the said parties hereunto set their hands and seals this
16th day of October, 2003

     /s/ S.K. Whoriskey                      /s/ Illegible
--------------------------------------  ----------------------------------------
LESSEE  Momenta Pharmaceuticals, Inc.   LESSOR  68 Moulton Street Realty Trust

--------------------------------------  ----------------------------------------
LESSEE                                  LESSOR

                      ------------------------------------------------
                      BROKER(S) Spaulding & Slye Co., Inc.

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                            RIDER TO COMMERCIAL LEASE

                    68 MOULTON STREET REALTY TRUST ("LESSOR")
                    MOMENTA PHARMACEUTICALS, INC. ("LESSEE")
              PREMISES: 68 MOULTON STREET, CAMBRIDGE, MASSACHUSETTS

A.      PARAGRAPH 4 CONTINUED.

        Included with the leased premises, at no extra cost to LESSEE, are 12
        outdoor parking spaces.

B.      PARAGRAPH 7 CONTINUED.

        For the avoidance of doubt, LESSOR and LESSEE have agreed that (i)
        normal business hours are 7am-7pm Monday through Friday and 10am-3pm
        Saturdays and Sundays; (ii) LESSOR will provide a cleaning. service
        Monday through Friday evenings to clean the leased premises and common
        areas (e.g. dust, vacuum, empty wastebaskets, clean the lavatories);
        (iii) LESSOR will be responsible for minor maintenance repairs (e.g.
        light bulb replacement, etc.), such services to typically be performed
        in the evening or on weekends; and (iv) LESSOR is providing all
        utilities typical for normal, general office space.

C.      PARAGRAPH 11 CONTINUED.

        Subsection 11(a) shall be modified by (i) inserting the words "(but only
        in the event such replacement is required as a result of LESSEE's
        negligence or willful misconduct)" after the words to "replace plate
        glass and other glass therein." and (ii) inserting the word "generally
        (subject to the last sentence of 11(b) below)" after "now" and before
        "in".

        Subsection 11(b) shall be supplemented with the following:
        "Notwithstanding anything herein to the contrary, LESSOR shall repair,
        replace and maintain in good condition and order the roof (including the
        roof membrane), walls, foundations, floor slabs and other structural
        elements of the building, and the HVAC, plumbing, electrical, mechanical
        and other systems serving the leased premises and the building. In
        addition, LESSOR agrees to patch and repair the damaged walls in the
        conference room and large office space and thoroughly vacuum the floors
        prior to LESSEE's occupancy of the leased premises.

E.      PARAGRAPH 12 CONTINUED.

        LESSOR agrees that LESSEE will be wiring the premises for computer,
        telecom and security.

F.      PARAGRAPH 13 CONTINUED.

        LESSOR agrees that it shall not unreasonably withhold, condition or
        delay its consent to an assignment or sublet of the whole or any part of
        the leased premises. Notwithstanding the provisions of Paragraph 13 to
        the contrary, LESSOR's consent shall not be required for any assignment
        or sublet to an entity controlling, controlled by, in common control

                                                                               1
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        with LESSEE, nor to any entity that succeeds to LESSEE's interest in
        this lease by reason of merger, sale/acquisition (whether by reason of
        the sale/acquisition of all or substantially all of the stock or
        assets), consolidation or reorganization; provided, however, such
        successor entity shall have a net worth comparable to LESSEE as of the
        date of such assignment and/or sublet or the date of execution of this
        lease, whichever is less.

G.      PARAGRAPH 15, CONTINUED.

        Notwithstanding the foregoing it is expressly agreed that unless
        otherwise agreed in writing, (i) all repairs and alterations shall be
        performed after normal business hours unless such repair or alteration
        is an emergency repair or alteration, and in any event LESSOR shall use
        commercially reasonable efforts to avoid disruption to LESSEE's business
        operations and (ii) LESSOR's right to view or show the leased premises,
        or to permit an agent to do so, shall be subject to a minimum of
        twenty-four (24) hours notice to LESSEE and in any event LESSOR shall
        use commercially reasonable efforts to avoid disruption to LESSEE's
        business operations.

H.      PARAGRAPH 16, CONTINUED.

        The first sentence of Paragraph 16 shall be stricken in its entirety and
        replaced with the following: "The LESSEE shall save the LESSOR harmless
        from all loss and damage occasioned by anything occurring on, the leased
        premises to the extent such loss and damage is caused by LESSEE or
        LESSEE's invitees, excluding any loss or damage caused by the negligence
        or misconduct of the LESSOR."

I.      PARAGRAPH 18, CONTINUED.

        In subsection (b), "ninety (90)" days shall be replaced with "thirty
        (30) days".

J.      PARAGRAPH 19, CONTINUED.

        "18%" shall be replaced with "10%".

K.      CASUALTY INSURANCE.

        LESSOR shall carry at all times during the term of the lease with
        respect to the leased premises and the property of which the leased
        premises are a part, insurance against loss or damage from fire and
        other casualties ordinarily included in a standard extended coverage or
        "all risks" endorsement. Such insurance shall be in an amount equal to
        at least the replacement value of said property, issued by companies
        qualified to do business in Massachusetts.

M.      WAIVER OF SUBROGATION.

        Lessor and Lessee will each use its best efforts to cause all policies
        of fire, extended coverage, and other physical damage insurance covering
        the Premises, the Building and any property therein to contain the
        insurers' waiver of subrogation and consent to pre-

                                                                               2
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        loss waiver of rights over by the insured. Notwithstanding any
        provisions of this lease to the contrary, Lessor and Lessee respectively
        waive all claims and rights to recover against the other for injury or
        loss due to hazards covered by insurance containing said clause or
        endorsement to the extent of the injury or loss covered thereby.

        If such provision or waiver is not obtainable without the payment of an
        additional premium, the party shall be relieved of its obligation to
        obtain or continue such provision or waiver in its policy or policies
        unless, in the case in which the same is obtainable upon payment of an
        additional premium, the other party pays such additional premium.

AGREED:

LESSEE:
Momenta Pharmaceuticals, Inc.

By:      /s/ S.K. Whoriskey
   -----------------------------------------

LESSOR:
68 Moulton Street Realty Trust

By:      /s/ Illegible
  ------------------------------------------

                                                                               3
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                               EXTENSION OF LEASE

     It is agreed between Momenta Pharmaceuticals, Inc., LESSEE, and 68 Moulton
Street Realty Trust, LESSOR, that the lease for four thousand, three hundred and
sixteen (4,316) square feet of office space located on the third floor of 68
Moulton Street, Cambridge, MA 02138, and dated October 16, 2003 be extended for-
a period of two (2) months commencing July 1, 2004 and ending August 31, 2004.

LESSEE:

Momenta Pharmaceuticals, Inc.

By:     /s/ Richard P. Shea
   -----------------------------------------

Date:   February 6, 2004
     ---------------------------------------

LESSOR:

68 Moulton Street Realty Trust By:

By:     /s/ Illegible
   -----------------------------------------

Date:   February 11, 2004
     ---------------------------------------

                                                                               4<Page>

                                                                   EXHIBIT 10.26

                         STANDARD FORM COMMERCIAL LEASE

1.  PARTIES                        68 Moulton Street Realty Trust, a
    (FILL IN)                      Massachusetts Trust with its mailing address
                                   of P.O. Box 95, Winchester, MA 01890. LESSOR,
                                   which expression shall include their heirs,
                                   successors, an assigns where the context so
                                   admits, does hereby lease to Momenta
                                   Pharmaceuticals, Inc., a Massachusetts
                                   corporation with its principal place of
                                   business at 43 Moulton Street, Cambridge, MA
                                   02138.

                                   LESSEE, which expression shall include
                                   its successors, executors, administrators,
                                   and assigns where the context so admits, and
                                   the LESSEE hereby leases the following
                                   described premises:

2.  PREMISES                       4,316 square feet of office space located on
    (FILL IN AND INCLUDE, IF       the second floor of 68 Moulton Street,
    APPLICABLE, SUITE              Cambridge, MA 02138, together with the right
    NUMBER, FLOOR NUMBER,          to use in common, with others entitled
    AND SQUARE FEET)               thereto, the hallways, stairways, and
                                   elevators, necessary for access to said
                                   leased premises, and lavatories nearest
                                   thereto.

3.  TERM                           The term of this lease shall be for seven (7)
    (FILL IN)                      months commencing on February 1, 2004 and
                                   ending on August 31, 2004.

4.  RENT                           The LESSEE shall pay to the LESSOR fixed rent
    (FILL IN)                      at the rate of $54,129.81 for a 7 month
                                   period payable in advance in monthly
                                   installments of 7,732.83 subject to proration
                                   in the case of any partial calendar month.
                                   All rent shall be payable without offset or
                                   deduction. Rent is calculated on the rate of
                                   $20.00 per square foot with an additional
                                   charge of $1.50 per square foot for
                                   electricity

5.  SECURITY                       Upon the execution of this lease, the LESSEE
    DEPOSIT                        shall pay to the LESSOR the amount of
    (FILL IN)                      $15,465.66 dollars, which shall be held as a
                                   security for the LESSEE's performance as
                                   herein provided and refunded to the LESSEE at
                                   the end of this lease, without interest,
                                   subject to the LESSEE's satisfactory
                                   compliance with the conditions hereof.

6.  RENT
    ADJUSTMENT

    A. TAX
       ESCALATION
       (FILL IN OR DELETE)

<Page>

    B. OPERATING
       COST
       ESCALATION
       (FILL IN OR DELETE)

    C. CONSUMER
       PRICE
       ESCALATION
       (FILL IN OR DELETE)

7.  UTILITIES                      The LESSOR agrees to provide all utility
                                   service and to furnish reasonably hot and
    *DELETE "AIR                   cold water and reasonable heat and air
    CONDITIONING" IF NOT           conditioning to the leased premises, the
    APPLICABLE                     hallways, stairways, elevators, and
                                   lavatories during normal business hours on
                                   regular business days of the heating and air
                                   conditioning seasons of each year, to furnish
                                   elevator service and to light passageways and
                                   stairways during business hours, and to
                                   furnish such cleaning service as is customary
                                   in similar buildings in said city or town,
                                   all subject to interruption due to any
                                   accident, to the making of repairs,
                                   alterations, or improvements, to labor
                                   difficulties, to trouble in obtaining fuel,
                                   electricity, service or supplies from the
                                   sources from which they are usually obtained
                                   for said building, or to any cause beyond the
                                   LESSOR's control.

                                   LESSOR shall have no obligation to provide
                                   utilities or equipment other than the
                                   utilities and equipment within the premises
                                   as of the commencement date of this lease. In
                                   the event LESSEE requires additional
                                   utilities or equipment, the installation and
                                   maintenance thereof shall be the LESSEE's
                                   sole obligation, provided that such
                                   installation shall be subject to the written
                                   consent of the LESSOR.

8.  USE OF LEASED                  The LESSEE shall use the leased premises only
    PREMISES                       for the purposes of General office
    (FILL IN)                      activities.

9.  COMPLIANCE                     The LESSEE acknowledges that no trade or
    WITH LAWS                      occupation shall be conducted in the leased
                                   premises or use made thereof which will be
                                   unlawful, improper, noisy or offensive, or
                                   contrary to any law or any municipal by-law
                                   or ordinance in force in the city or town in
                                   which the premises are situated. Without
                                   limiting the generality of the foregoing (a)
                                   the LESSEE shall not bring or permit to be
                                   brought or kept in or on the leased premises
                                   or elsewhere on the LESSOR's property any
                                   hazardous, toxic, inflammable, combustible or
                                   explosive fluid, material, chemical or
                                   substance, including without limitation any
                                   item defined as hazardous pursuant to Chapter
                                   21E of the Massachusetts General laws; and
                                   (b) the LESSEE shall be responsible for
                                   compliance

<Page>

                                   with requirements imposed by the American
                                   with Disabilities Act relative to the layout
                                   of the leased premises and any work performed
                                   by the LESSEE therein.

10. FIRE INSURANCE                 The LESSEE shall not permit any use of the
                                   leased premises which will make voidable any
                                   insurance on the property of which the leased
                                   premises are a part, or on the contents of
                                   said property or which shall be contrary to
                                   any law or regulation from time to time
                                   established by the New England Fire Insurance
                                   Rating Association, or any similar body
                                   succeeding to its powers. The LESSEE shall on
                                   demand reimburse the LESSOR, and all other
                                   tenants, all extra insurance premiums caused
                                   by the LESSEE's use of the premises.

11. MAINTENANCE                    The LESSEE agrees to maintain the leased
                                   premises in good condition, damage by fire
    A. LESSEE'S                    and other casualty only excepted, and
       OBLIGATIONS                 whenever necessary, to replace plate glass
                                   and other glass therein, acknowledging that
                                   the leased premises are now in order and the
                                   glass whole. The LESSEE shall not permit the
                                   leased premises to be overloaded, damaged,
                                   stripped, or defaced, nor suffer any waste.
                                   LESSEE shall obtain written consent of LESSOR
                                   before erecting any sign on the premises.

    B. LESSOR'S                    The LESSOR agrees to maintain the structure
       OBLIGATIONS                 of the building of which the leased premises
                                   are a part in the same condition as it is at
                                   the commencement of the term or as it may be
                                   put in during the term of this lease,
                                   reasonable wear and tear, damage by fire and
                                   other casualty only excepted, unless such
                                   maintenance is required because of the LESSEE
                                   or those for whose conduct the LESSEE is
                                   legally responsible.

12. ALTERATIONS -                  The LESSEE shall not make structural
    ADDITIONS                      alterations or additions to the leased
                                   premises, but may make non-structural
                                   alterations provided the LESSOR consents
                                   thereto in writing, which consent shall not
                                   be unreasonably withheld or delayed. All such
                                   allowed alterations shall be at LESSEE's
                                   expense and shall be in quality at least
                                   equal to the present construction. LESSEE
                                   shall not permit any mechanics' liens, or
                                   similar liens, to remain upon the leased
                                   premises for labor and material furnished to
                                   LESSEE or claimed to have been performed at
                                   the direction of LESSEE an shall cause any
                                   such lien to be released of record forthwith
                                   without cost to LESSOR. Any alterations or
                                   improvements made by the LESSEE shall become
                                   the property of the LESSOR at the termination
                                   of occupancy as provided herein.

13. ASSIGNMENT -                   The LESSEE shall not assign or sublet the
    SUBLEASING                     whole or any part of the leased premises
                                   without LESSOR's prior written consent.
                                   Notwithstanding such consent, LESSEE shall
                                   remain liable to

<Page>

                                   LESSOR for the payment of all rent and for
                                   the full performance of the covenants and
                                   conditions of this lease.

14. SUBORDINATION                  This lease shall be subject and subordinate
                                   to any and all mortgages, deeds of trust and
                                   other instruments in the nature of a
                                   mortgage, now or at any time hereafter, a
                                   lien or liens on the property of which the
                                   leased premises are a part and the LESSEE
                                   shall, when requested, promptly execute and
                                   deliver such written instruments as shall be
                                   necessary to show the subordination of this
                                   lease to said mortgages, deeds of trust or
                                   other such instruments in the nature of a
                                   mortgage.

15. LESSOR'S                       The LESSOR or agents of the LESSOR may, at
    ACCESS                         reasonable times, enter to view the leased
                                   premises and may remove placards and signs
                                   not approved and affixed as herein provided,
                                   and make repairs and alterations as LESSOR
                                   should elect to do and may show the leased
                                   premises to others, and at any time affix to
                                   any suitable part of the leased premises a
                                   notice for letting or selling the leased
                                   premises or property of which the leased
                                   premises are a part and keep the same so
                                   affixed without hindrance or molestation.

16. INDEMNIFICATION                The LESSEE shall save the LESSOR harmless
    AND LIABILITY                  from all loss and damage occasioned by
    (FILL IN)                      anything occurring on the leased premises
                                   unless caused by the negligence or misconduct
                                   of the LESSOR, and from all loss and damage
                                   wherever occurring occasioned by any
                                   omission, fault, neglect or other misconduct
                                   of the LESSEE. The removal of snow and ice
                                   from the sidewalks bordering upon the leased
                                   premises shall be Lessor's responsibility.

17. LESSEE'S                       The LESSEE shall maintain with respect to the
    LIABILITY                      leased premises and the property of which the
    INSURANCE                      leased premises are a part comprehensive
    (FILL IN)                      public liability insurance in the amount of
                                   $2,000,000.00 with property damage insurance
                                   in limits of $2,000,000.00 in responsible
                                   companies qualified to do business in
                                   Massachusetts and in good standing therein
                                   insuring the LESSOR as well as LESSEE against
                                   injury to persons or damage to property as
                                   provided. The LESSEE shall deposit within the
                                   LESSOR certificates for such insurance at or
                                   prior to the commencement of the term, and
                                   thereafter within thirty (30) days prior to
                                   the expiration of any such policies. All such
                                   insurance certificates shall provide that
                                   such policies shall not be cancelled without
                                   at least ten (10) days prior written notice
                                   to each assured named therein.

18. FIRE, CASUALTY -               Should a substantial portion of the leased
    EMINENT DOMAIN                 premises, or of the property of which they
                                   are a part, be substantially damaged by fire
                                   or other casualty, or be taken by eminent
                                   domain, the

<Page>

                                   LESSOR may elect to terminate this lease.
                                   When such fire, casualty, or taking renders
                                   the leased premises substantially unsuitable
                                   for their intended uses, a just and
                                   proportionate abatement of rent shall be
                                   made, and the LESSEE may elect to terminate
                                   this lease if:

                                        (a)  The LESSOR fails to give written
                                             notice within thirty (30) days of
                                             intention to restore leased
                                             premises, or

                                        (b)  The LESSOR fails to restore the
                                             leased premises to a condition
                                             substantially suitable for their
                                             intended use within ninety (90)
                                             days of said fire, casualty or
                                             taking.

                                   The LESSOR reserves, and the LESSEE grants to
                                   the LESSOR, all rights which the LESSEE may
                                   have for damages or injury to the leased
                                   premises for any taking by eminent domain,
                                   except for damage to the LESSEE's fixtures,
                                   property, or equipment.

19. DEFAULT AND                    In the event that:
    BANKRUPTCY
    (FILL IN)                           (a)  The LESSEE shall default in the
                                             payment of any installment of rent
                                             or other sum herein specified and
                                             such default shall continue for ten
                                             (10) days after written notice
                                             thereof; or

                                        (b)  The LESSEE shall default in the
                                             observance or performance of any
                                             other of the LESSEE's covenants,
                                             agreements, or obligations
                                             hereunder and such default shall
                                             not be corrected within thirty (30)
                                             days after written notice thereof;
                                             or

                                        (c)  The LESSEE shall be declared
                                             bankrupt or insolvent according to
                                             law, or, if any assignment shall be
                                             made of LESSEE's property for the
                                             benefit of creditors.

                                   then the LESSOR shall have the right
                                   thereafter, while such default continues, to
                                   re-enter and take complete possession of the
                                   leased premises, to declare the term of this
                                   lease ended, and remove the LESSEE's effects,
                                   without prejudice to any remedies which might
                                   be otherwise used for arrears of rent or
                                   other default. The LESSEE shall indemnify the
                                   LESSOR against all loss of rent and other
                                   payments which the LESSOR may incur by reason
                                   of such termination during the residue of the
                                   term. If the LESSEE shall default, after
                                   reasonable notice thereof, in the observance
                                   or performance of any conditions or covenants
                                   on LESSEE's part to be observed or performed
                                   under or by virtue of any of the provisions
                                   in any article of this lease, the LESSOR,
                                   without being under any obligation to do so
                                   and without thereby waiving such default, may
                                   remedy such default for the account

<Page>

                                   and at the expense of the LESSEE. If the
                                   LESSOR makes any expenditures or incurs any
                                   obligations for the payment of money in
                                   connection therewith, including but not
                                   limited to, reasonable attorney's fees in
                                   instituting, prosecuting or defending any
                                   action or proceeding, such sums paid or
                                   obligations insured, with interest at the
                                   rate of 18% percent per annum and costs,
                                   shall be paid to the LESSOR by the LESSEE as
                                   additional rent.

20. NOTICE                         Any notice from the LESSOR to the LESSEE
    (FILL IN)                      relating to the leased premises or to the
                                   occupancy thereof, shall be deemed duly
                                   served, if left at the leased premises
                                   addressed to the LESSEE, or if mailed to the
                                   leased premises, registered or certified
                                   mail, return receipt requested, postage
                                   prepaid, addressed to the LESSEE. Any notice
                                   from the LESSEE to the LESSOR relating to the
                                   leased premises or to the occupancy thereof,
                                   shall be deemed duly served, if mailed to the
                                   LESSOR by registered or certified mail,
                                   return receipt requested, postage prepaid,
                                   addressed to the LESSOR at such address as
                                   the LESSOR may from time to time advise in
                                   writing. All rent notices shall be paid and
                                   sent to the LESSOR at P.O. Box 95,
                                   Winchester, MA 01890.

21. SURRENDER                      The LESSEE shall at the expiration or other
                                   termination of this lease remove all LESSEE's
                                   goods and effects from the leased premises,
                                   (including, without hereby limiting the
                                   generality of the foregoing, all signs and
                                   lettering affixed or painted by the LESSEE,
                                   either inside or outside the leased
                                   premises). LESSEE shall deliver to the LESSOR
                                   the leased premises and all keys, locks
                                   thereto, and other fixtures connected
                                   therewith and all alterations and additions
                                   made to or upon the leased premises, in good
                                   condition, damage by fire or other casualty
                                   only excepted. In the event of the LESSEE's
                                   failure to remove any of LESSEE's property
                                   from the premises, LESSOR is hereby
                                   authorized, without liability to LESSEE for
                                   loss or damage thereto, and at the sole risk
                                   of LESSEE, to remove and store any of the
                                   property at LESSEE's expense, or to retain
                                   same under LESSOR's control or to sell at
                                   public or private sale, without notice any or
                                   all of the property not so removed and to
                                   apply the net proceeds of such sale to the
                                   payment of any sum due hereunder, or to
                                   destroy such property.

22.
    (FILL IN OR DELETE)

23. CONDITION OF                   Except as may be otherwise expressly set
    PREMISES                       forth herein, the LESSEE shall accept the
                                   leased premises "as is" in their condition as
                                   of the commencement of the term of this
                                   lease, and the LESSOR shall be obligated to
                                   perform no work whatsoever in order to
                                   prepare the leased premises for occupancy by
                                   the

<Page>

                                   LESSEE.

24. FORCE                          In the event that the LESSOR is prevented or
    MAJEURE                        delayed from making any repairs or performing
                                   any other covenant hereunder by reason of any
                                   cause reasonably beyond the control of the
                                   LESSOR, the LESSOR shall not be liable to the
                                   LESSEE therefore nor, except as expressly
                                   otherwise provided in case of casually or
                                   taking, shall the LESSEE be entitled to any
                                   abatement or reduction of rent by reason
                                   thereof, nor shall the same give rise to a
                                   claim by the LESSEE that such failure
                                   constitutes actual or constructive eviction
                                   from the leased premises or any part thereof.

25. LATE                           If rent or any other sum payable hereunder
    CHARGE                         remains outstanding for a period of ten (10)
                                   days, the LESSEE shall pay to the LESSOR a
                                   late charge equal to one and one-half percent
                                   (1.5%) of the amount due for each month or
                                   portion thereof during which the arrearage
                                   continues.

26. LIABILITY                      No owner of the property of which the leased
    OF OWNER                       premises are a part shall be liable hereunder
                                   except for breaches of the LESSOR's
                                   obligations occurring during the period of
                                   such ownership. The obligations of the LESSOR
                                   shall be binding upon the LESSOR's interest
                                   in said property, but not upon other assets
                                   of the LESSOR, and no individual partner,
                                   agent, trustee, stockholder, officer,
                                   director, employee or beneficiary of the
                                   LESSOR shall be personally liable for
                                   performance of the LESSOR's obligations
                                   hereunder.

27. OTHER
    PROVISIONS

IN WITNESS WHEREOF, the said parties hereunto set their hands and seals this 11
day of February, 2004.

     /s/ Richard P. Shea                       /s/ illegible
---------------------------------------    -------------------------------------
LESSEE Momenta Pharmaceutical, Inc.        LESSOR 68 Moulton Street Realty Trust

---------------------------------------    -------------------------------------
LESSEE                                     LESSOR

<Page>

                            RIDER TO COMMERCIAL LEASE

                    68 MOULTON STREET REALTY TRUST ("LESSOR")
                    MOMENTA PHARMACEUTICALS, INC. ("LESSEE")
              PREMISES: 68 MOULTON STREET, CAMBRIDGE, MASSACHUSETTS

A.   PARAGRAPH 4, CONTINUED

     Included with the leased premises, at no extra cost to LESSEE, are 12
     outdoor Parking spaces.

B.   PARAGRAPH 7, CONTINUED

     For the avoidance of doubt, LESSOR and LESSEE have agreed that (i) normal
     business hours at 7am-7pm Monday through Friday and 10am-3pm Saturdays an
     Sundays; (ii) LESSOR will provide a cleaning service Monday through Friday
     evenings to clean the leased premises and common areas (e.g. dust, vacuum,
     empty wastebaskets, clean the lavatories); (iii) LESSOR will be responsible
     for minor maintenance repairs (e.g. light bulb replacement, etc.), such
     services to typically be performed in the evenings or on weekends; and (iv)
     LESSOR is providing all utilities typical for normal, general office space.

C.   PARAGRAPH 11, CONTINUED

     Subsection 11(a) shall be modified (i) inserting words "(but only in the
     event such replacement is required as a result of LESSEE's negligence or
     willful misconduct)" after the words to "replace plate glass and other
     glass therein" and (ii) inserting the words "generally (subject to the last
     sentence of 11(b) below)" after "now" and before "in".

     Subsection 11(b) shall be supplemented with the following "Notwithstanding
     anything herein to the contrary, LESSOR shall repair, replace and maintain
     in good condition and order the roof (including the roof membrane), walls,
     foundations, floor slab and other structural elements of the building, and
     the HVAC, plumbing, electrical, mechanical and other systems serving the
     leased premises and the building.

E.   PARAGRAPH 12, CONTINUED

     LESSOR agrees that LESSEE will be wiring the premises for computer, telecom
     and security.

F.   PARAGRAPH 13, CONTINUED

     LESSOR agrees that it shall not unreasonably withhold, condition or delay
     its consent to an assignment or sublet of the whole or any part of the
     leased premises. Notwithstanding the provisions of Paragraph 13 to the
     contrary, LESSOR's consent shall not be required for any assignment or
     sublet to an entity controlling, controlled by, in common control with
     LESSEE, nor to any entity that succeeds to LESSEE's interest in this lease
     by reason of merger, sale/acquisition (whether by consolidation or
     reorganization; provided, however, such successor entity shall have a net
     worth comparable to LESSEE as of the

<Page>

     date of such assignment and/or sublet or the date of execution of this
     lease, whichever is less.

G.   PARAGRAPH 15, CONTINUED

     Notwithstanding the foregoing it is expressly agreed that unless otherwise
     agreed in writing, (i) all repairs and alterations shall be performed after
     normal business hours unless such repair or alteration is an emergency
     repair or alteration, and in any event LESSOR shall use commercially
     reasonable efforts to avoid disruptions to LESSEE's business operations and
     (ii) LESSOR's right to view or show the leased premises, or to permit an
     agent to do so, shall be subject to minimum of twenty-four (24) hours
     notice to LESSEE and in any event LESSOR shall use commercially reasonable
     efforts to avoid disruption to LESSEE's business operations.

H.   PARAGRAPH 16, CONTINUED

     The first sentence of Paragraph 16 shall be stricken in its entirety and
     replaced with the following: "The LESSEE shall save the LESSOR harmless
     from all loss and damage occasioned by anything occurring on the leased
     premises to the extent such loss and damage is caused by LESSEE or LESSEE's
     invitees, excluding any loss or damage caused by the negligence or
     misconduct of the LESSOR."

I.   PARAGRAPH 18, CONTINUED

     In subsection (b), "ninety (90)" days shall be replaced with "thirty (30)
     days".

J.   PARAGRAPH 19, CONTINUED

     "18%" shall be replaced with "10%".

K.   CASUALTY INSURANCE.

     LESSOR shall carry at all times during the term of the lease with respect
     to the leased premises and the property of which the leased premises are a
     part, insurance against loss or damage from fire and other casualties
     ordinarily included in a standard extended coverage or "all risks"
     endorsement. Such insurance shall be in an amount equal to at least the
     replacement value of said property, issued by companies qualified to do
     business in Massachusetts.

M.   WAIVER OF SUBROGATION.

     LESSOR and LESSEE will each use its best efforts to cause all policies of
     fire, extended coverage, and other physical damage insurance covering the
     Premises, the Building and any property therein to contain the insurers'
     waiver of subrogation and consent to pre-loss waiver of rights over by the
     insured. Notwithstanding any provisions of this lease to the contrary,
     LESSOR and LESSEE respectively waive all claims and rights to recover
     against the other for injury or loss due to hazards covered by insurance
     containing said clause or endorsement to the extent of the injury or loss
     covered thereby.

<Page>

     If such provision or waiver is not obtainable without the payment of an
     additional premium, the party shall be relieved of its obligation to obtain
     or continue such provision or waiver in its policy or policies unless, in
     the case in which the same is obtainable upon payment of an additional
     premium, the other party pays such additional premium.

AGREED

LESSEE:
MOMENTA PHARMACEUTICALS, INC.

BY:   /s/ RICHARD P. SHEA
   ----------------------------

LESSOR:
68 MOULTON STREET REALTY TRUST

BY:   /s/ Illegible
   ----------------------------

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