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                                                                   Exhibit 10.6

                                 LEASE AGREEMENT

         THIS TRIPLE NET LEASE AGREEMENT (the "LEASE") is entered into this 11th
day of July, 2000, between 830 WINTER STREET LLC, a Delaware limited liability
company ("LANDLORD"), and PRAECIS PHARMACEUTICALS INCORPORATED, a Delaware
corporation ("TENANT"). Landlord and Tenant agree as follows:

1.   LEASE SUMMARY.

     a. LEASED PREMISES. The leased commercial real estate (the "PREMISES")
     consist of the real property located at 830 Winter Street, Waltham,
     Massachusetts, legally described on attached Exhibit A, and all
     improvements thereon.

     b. LEASE COMMENCEMENT DATE. The Lease shall commence on July 11, 2000, or
     such later date as of which title to the Premises shall have vested in the
     Landlord (the "COMMENCEMENT DATE").

     c. TERMINATION DATE. The Lease shall terminate at midnight on the date
     immediately preceding the fifteenth anniversary of the Lease Commencement
     Date, or if such date is not the last date of the calendar month then the
     last day of the calendar month in which such date shall occur (the
     "TERMINATION DATE").

     d. BASE RENT. The base monthly rent shall be as set forth on SCHEDULE 1(d),
     hereof. Rent shall be payable at Landlord's address shown in Section 1(g)
     below, or such other place designated in writing by Landlord.

     e. SECURITY DEPOSIT. None.

     f. PERMITTED USE. The Premises shall be used for business and professional
     offices, and for research laboratories and related ancillary uses.

     g. NOTICE AND PAYMENT ADDRESSEE:
        Landlord: 830 WINTER STREET LLC
                  c/o PRAECIS PHARMACEUTICALS INCORPORATED
                  One Hampshire Street
                  Cambridge, MA 02139
                  Tel: (617) 494-8400
                  Fax: (617) 621-8679

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     Tenant: PRAECIS PHARMACEUTICALS INCORPORATED
                      One Hampshire Street
                      Cambridge, MA 02139
                      Tel: (617) 494-8400
                      Fax: (617) 621-8679

     h. EXTENSION OPTION: Tenant may elect to extend the term beyond the
     expiration of the original term of this Lease for one (1) additional period
     of five (5) years each under Section 2.b.

     i. TENANT'S WORK ALLOWANCE: $9,000,000.00.

2.   PREMISES. Landlord leases to Tenant, and Tenant leases from Landlord the
     Premises upon the terms specified in this Lease.

3. TERM.

     a. COMMENCEMENT DATE. The Lease shall commence on the date specified in
     Section 1(b). The first "Lease Year" shall commence on the Commencement
     Date and shall end on the date which is twelve (12) months from the end of
     the month in which the Commencement Date occurs. Each successive Lease year
     during the initial term and any extension terms shall be twelve (12)
     months, commencing on the first day following the end of the preceding
     Lease Year, except that the last Lease Year shall end on the Termination
     Date.

     b. EXTENSION TERM. Provided that no Event of Default with regard to the
     payment of Rent is then in existence, Tenant shall have the right to extend
     the original term of this Lease for one (1) period of five (5) years. Such
     option to extend shall be exercised by the giving of notice by Tenant to
     Landlord on or before the date which is twelve (12) months prior to the
     expiration of the then current Term. Upon the giving of such notice, this
     Lease and the Term hereof shall be extended, for an additional term of five
     (5) years (the "Extension Term"), without the necessity for the execution
     of any additional documents, provided that upon request of either party,
     Landlord and Tenant agree to enter into an amendment of this Lease to
     confirm the exercise of such extension option and to document all changes
     to the Lease resulting from the exercise of the extension option. Time is
     of the essence in the giving of such notice. The Extension Term shall be
     upon all the terms, conditions and provisions of this Lease except that the
     Rent during the Extension Term ("Extension Term Rent") shall be equal to
     the fair rental value of the Premises for such Extension Term as reasonably
     determined by Landlord and Tenant.

     c. TENANT OBLIGATIONS. Except as specified elsewhere in this Lease,
     Landlord makes no representations or warranties to Tenant regarding the
     Premises, including the structural condition of the Premises and the
     condition of all mechanical, electrical, and other systems on the Premises.
     Tenant shall be responsible for performing any work necessary to bring the
     Premises into condition satisfactory to Tenant. By signing this

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     Lease, Tenant acknowledges that it has had adequate opportunity to
     investigate the Premises, acknowledges responsibility for making any
     corrections, alterations and repairs to the Premises, and acknowledges that
     the time needed to complete any such items shall not delay the Commencement
     Date.

4.   RENT. Tenant shall pay Landlord without demand, deduction or offset, in
     lawful money of the United States, the monthly rental stated in SCHEDULE
     1(d) in advance on or before the first day of each month during the Lease
     Term, and any other additional payments due to Landlord (collectively the
     "RENT") when required under this Lease. Payments for any partial month at
     the beginning or end of the Lease term shall be prorated.

     If any sums payable by Tenant to Landlord under this Lease are not received
     by the tenth (10th) day of each month, Tenant shall pay Landlord in
     addition to the amount due, for the cost of collecting and handling such
     late payment, an amount equal to the greater of $100 or five percent (5%)
     of the delinquent amount. In addition, all delinquent sums payable by
     Tenant to Landlord and not paid within ten (10) days of the due date shall,
     at Landlord's option, bear interest at the rate of fourteen percent (14%)
     per annum, or the highest rate of interest allowable by law, whichever is
     less. Interest on all delinquent amounts shall be calculated from the
     original due date to the date of payment.

     Landlord's acceptance of less than the full amount of any payment due from
     Tenant shall not be deemed an accord and satisfaction or compromise of such
     payment unless landlord specifically consents in writing to payment of such
     lesser sum as an accord and satisfaction or compromise of the amount which
     Landlord claims.

5.   SECURITY DEPOSIT. There shall be no security deposit required.

6.   USES. The Premises shall be used only for the use(s) specified in Section
     1(f) above (the "PERMITTED USES"), and for no other business or purpose
     without the prior written consent of Landlord. No act shall be done on or
     around the Premises that is unlawful or that will increase the existing
     rate of insurance on the Premises. Tenant shall not commit or allow to be
     committed any waste upon the Premises, or any public or private nuisance.

7.   COMPLIANCE WITH LAWS. Tenant shall not cause or permit the Premises to be
     used in any way which violates any law, ordinance, or governmental
     regulation or order. Tenant shall be responsible for complying with all
     laws applicable to the Premises as a result of Tenant's particular use,
     such as modifications required by the Americans With Disabilities Act as a
     result of Tenant opening the Premises to the public as a place of public
     accommodation. If the enactment or enforcement of any law, ordinance,
     regulation or code during the Lease term requires any changes to the
     Premises during the Lease term, the Tenant shall perform all such changes
     at its

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     expense if the changes are required due to the nature of Tenant's
     activities at the Premises, or to alterations that Tenant seeks to make to
     the Premises; otherwise, Landlord shall perform all such changes at its
     expense.

8.   UTILITIES. Landlord shall not be responsible for providing any utilities to
     the Premises, but represents and warrants to Tenant that as of the
     Commencement Date electricity, water, sewer, and telephone utilities are
     available at or adjacent to the Premises. Tenant shall determine whether
     the available capacity of such utilities will meet Tenant's needs. Tenant
     shall install and connect, if necessary, and directly pay for all water,
     sewer, gas, janitorial, electricity, garbage removal, heat, telephone, and
     other utilities and services used by Tenant on the Premises during the
     Term, whether or not such services are billed directly to Tenant. Tenant
     will also procure, or cause to be procured, without cost to Landlord, all
     necessary permits, licenses or other authorizations required for the lawful
     and proper installation, maintenance, replacement and removal on or from
     the Premises of wires, pipes, conduits, tubes, and other equipment and
     appliances for use in supplying all utilities or services to the Premises.
     Landlord, upon request of Tenant, and at the sole expense and liability of
     Tenant, shall join with Tenant in any application required for obtaining or
     continuing such utilities or services.

9.   TAXES. Tenant shall pay all Taxes directly to the applicable taxing
     authority at least five days prior to the date such taxes are due and
     payable. Upon request of Landlord, Tenant shall supply Landlord with copies
     of receipt of payment of taxes or other proof reasonably satisfactory to
     Landlord showing the payment of Taxes.

     The term "TAXES" shall mean: (i) any form of real estate tax or assessment
     imposed on the Premises by any authority, including any city, state or
     federal government, or any improvement district, as against any legal or
     equitable interest of Landlord or Tenant in the Premises or in the real
     property of which the Premises are a part, or against rent paid for leasing
     the Premises; and (ii) any form of personal property tax or assessment
     imposed on any personal property, fixtures, furniture, tenant improvements,
     equipment, inventory, or other items, and all replacements, improvements,
     and additions to them, located on the Premises, whether owned by Landlord
     or Tenant. "Taxes" shall exclude any net income tax imposed on Landlord for
     income that Landlord receives under this Lease.

     Tenant may contest the amount or validity, in whole or in part, of any
     Taxes at its sole expense, only after paying such Taxes or posting such
     security as Landlord may reasonably require in order to protect the
     premises against loss or forfeiture. Upon the termination of any such
     proceedings, Tenant shall pay the amount of such Taxes or part of such
     Taxes as finally determined, together with any costs, fees, interest
     penalties, or other related liabilities. Landlord shall cooperate with
     Tenant in contesting any Taxes, provided Landlord incurs no expense or
     liability in doing so.

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10.  ALTERATIONS. Tenant may make alterations, additions or improvements to the
     Premises ("ALTERATIONS"). Tenant shall complete all Alterations at Tenant's
     expense in compliance with all applicable laws and in accordance with plans
     and specifications approved by Landlord, and using contractors approved by
     Landlord provided, however, that Landlord shall make the Tenant's Work
     Allowance available to Tenant for Tenant's Work upon written request of
     Tenant. Landlord shall be deemed the owner of all Alterations except for
     those which Landlord requires to be removed at the end of the Lease term.

11.  REPAIRS AND MAINTENANCE. Subject to Section 14 Tenant shall, at its sole
     expense, maintain the Premises in good condition and promptly make all
     repairs and replacements necessary to keep the Premises in safe operating
     condition, including the roof, foundation, exterior walls and all utilities
     and other systems serving the Premises. Tenant shall not damage or disturb
     the structural integrity of the Premises and shall promptly repair any
     damage or injury done to any such demising walls or structural elements
     caused by Tenant or its employees, agents, contractors, or invitees.
     Notwithstanding anything in this Section to the contrary, Tenant shall not
     be responsible for any repairs to the Premises made necessary by the acts
     of Landlord or its agents, employees, contractors or invitees therein.

     Upon expiration of the Lease term, whether by lapse of time or otherwise,
     Tenant shall promptly and peacefully surrender the Premises, together with
     all keys, to Landlord in as good condition as when received by Tenant from
     Landlord or as thereafter improved, reasonable wear and tear and insured
     casualty excepted.

12.  ACCESS. Upon reasonable notice from Landlord (except in cases of emergency,
     where no notice in required), Tenant shall permit Landlord and its agents
     and employees to enter the Premises at all reasonable times for the
     purposes of repair or inspection. This Section shall not impose any repair
     or other obligation upon Landlord not expressly stated elsewhere in this
     Lease. After reasonable notice to Tenant, Landlord shall have the right to
     enter the Premises for the purpose of showing the Premises to prospective
     purchasers or lenders at any time, and to prospective tenants within 180
     days prior to the expiration or sooner termination of the Lease term.

13.  SIGNAGE. Tenant shall install any signage at Tenant's sole expense and in
     compliance with all applicable laws. Tenant shall not damage or deface the
     premises in installing or removing signage and shall repair any injury or
     damage to the Premises caused by such installation or removal.

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14.  DESTRUCTION OR CONDEMNATION.

     a. If the Premises are damaged or destroyed by fire or other casualty,
     Landlord shall, to the extent that insurance proceeds are made available to
     Landlord by mortgagee, restore the Premises and this Lease shall not
     terminate.

     b. In case of taking of a part of the Premises that does not render the
     Premises untenantable, then this Lease shall continue in full force and
     effect and this Lease shall not terminate.

15.  INSURANCE.

     a. During the Lease term, Tenant shall pay for and maintain insurance as
     set forth on SCHEDULE 15(a).

     b. WAIVER OF SUBROGATION. Landlord and Tenant hereby release each other and
     any other tenant, their agents or employees, from responsibility for, and
     waive their entire claim of recovery for any loss or damage arising from
     any cause covered by insurance required to be carried by each of them. Each
     party shall provide notice to the insurance carrier or carriers of this
     mutual waiver of subrogation, and shall cause its respective insurance
     carriers to waive all rights of subrogation against the other. This waiver
     shall not apply to the extent of the deductible amounts to any such
     policies or to the extent of liabilities exceeding the limits of such
     policies.

16.  INDEMNIFICATION. Tenant shall defend, indemnify, and hold Landlord harmless
     from and against all liabilities, damages, costs, and expenses, including
     attorneys' fees, arising from any negligent or wrongful act or omission of
     Tenant or Tenant's officers, contractors, licensees, agents, servants,
     employees, guests, invitees, or visitors on or around the Premises as a
     result of any act, omission or negligence of Tenant, or Tenant's officers,
     contractors, licensees, agents, servants, employees, guests, invitees, or
     visitors, or arising from any breach of this Lease by Tenant. Tenant shall
     use legal counsel acceptable to Landlord in defense of any action within
     Tenant's defense obligation.

17.  ASSIGNMENT AND SUBLETTING. Tenant may not assign or sublet any interest in
     this Lease or any part of the premises (collectively referred to as a
     "TRANSFER"), without obtaining Landlord's prior written consent, which
     consent shall be granted or withheld in Landlord's sole discretion. No
     Transfer shall relieve Tenant of any liability under this Lease.

     In connection with any Transfer, Tenant shall provide Landlord with copies
     of all assignments, subleases and assumption instruments.

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18.  LIENS. Tenant shall keep the Premises free from any liens created by or
     through Tenant. Tenant shall indemnify and hold Landlord harmless from
     liability from any such liens including, without limitation, liens arising
     from any Alterations. If a lien is filed against the Premises by any person
     claiming by, through or under Tenant, Tenant shall, upon request of
     Landlord, at Tenant's expense, promptly furnish to Landlord a bond in form
     and amount and issued by a surety satisfactory to Landlord, indemnifying
     Landlord and the Premises against all liabilities, costs and expenses,
     including attorneys; fees, which Landlord could reasonably incur as a
     result of such lien(s).

19.  DEFAULT. The following occurrences shall each be deemed an "Event of
     Default" by Tenant:

     a. FAILURE TO PAY. Tenant fails to pay any sum, including Rent, when due
     under this Lease.

     b. INSOLVENCY. Tenant becomes insolvent, voluntarily or involuntarily
     bankrupt, or a receiver, assignee or other liquidating officer is appointed
     for Tenant's business, provided that in the event of any involuntary
     bankruptcy or other insolvency proceeding, the existence of such proceeding
     such constitute an Event of Default only if such proceeding is not
     dismissed or vacated within 60 days after its institution or commencement.

     c. LEVY OR EXECUTION. Tenant's interest in this Lease or the Premises, or
     any part thereof, is taken by execution or other process of law directed
     against Tenant, or is taken upon or subjected to any attachment by any
     creditor of Tenant, if such attachment is not discharged within 15 days
     after being levied.

     d. OTHER NON-MONETARY DEFAULTS. Tenant breaches any agreement, term or
     covenant of this Lease other than one requiring the payment of money and
     not otherwise enumerated in this Section, and the breach continues for a
     period of 30 days after notice by Landlord to Tenant of the breach. Tenant
     shall not be considered in default if the cure of such breach reasonably
     takes longer than 30 days and Tenant has begun to diligently pursue a cure
     of such breach within the 30 day period and completes such cure within
     ninety (90) days thereafter.

20.  REMEDIES.

     Landlord shall have the following remedies upon an Event of Default.
     Landlord's rights and remedies under this Lease shall be cumulative, and
     none shall exclude any other right or remedy allowed by law.

     a. TERMINATION OF LEASE. Landlord may terminate Tenant's interest under the
     Lease, but no act by Landlord, other than written notice from Landlord to
     Tenant of

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     termination, shall terminate this Lease. The Lease shall terminate on the
     date specified in the notice of termination. Upon termination of this
     Lease, Tenant will remain liable to Landlord for damages in an amount equal
     to the rent and other sums that would have been owing by Tenant under this
     Lease for the balance of the Lease term, less the net proceeds, if any, of
     any reletting of the Premises by Landlord subsequent to the termination,
     after deducting all Landlord's Reletting Expenses (as defined below).
     Landlord shall be entitled to either collect damages from Tenant monthly on
     the days on which rent or other amounts would have been payable under the
     Lease, or alternatively, Landlord may accelerate Tenant's obligations under
     the Lease and recover from Tenant; (i) unpaid rent which had been earned at
     the time of termination; (ii) the amount by which the unpaid rent which
     would have been earned after termination until the time of award exceeds
     the amount of rent loss that Tenant proves could reasonably have been
     avoided; (iii) the amount by which the unpaid rent for the balance of the
     term of the Lease after the time of award exceeds the amount of rent loss
     that Tenant proves could reasonably be avoided (discounting such amount by
     the discount rate of the Federal Reserve Bank at the time of the award,
     plus 1%); and (iv) any other amount necessary to compensate Landlord for
     all the detriment proximately caused by Tenant's failure to perform its
     obligations under the Lease, or which in the ordinary course would be
     likely to result from the Event of Default, including without limitation
     Reletting Expenses described in Section 20b.

     b. RE-ENTRY AND RELETTING. Landlord may continue this Lease in full force
     and effect, and without demand or notice, re-enter and take possession of
     the Premises or any part thereof, expel the Tenant from the Premises and
     anyone claiming through or under the Tenant, and remove the personal
     property of either. Landlord may relet the Premises, or any part of them,
     in Landlord's or Tenant's name for the account of Tenant, for such period
     of time and at such other terms and conditions, as Landlord, in its
     discretion, may determine. Landlord may collect and receive the rents for
     the Premises. Re-entry or taking possession of the Premises by Landlord
     under this Section shall not be construed as an election on Landlord's part
     to terminate this Lease, unless a written notice of termination is given to
     Tenant. Landlord reserves the right following any re-entry or reletting, or
     both, under this Section to exercise its right to terminate the Lease.
     During the Event of Default, Tenant will pay Landlord the rent and other
     sums which would be payable under this Lease if repossession had not
     occurred, plus the net proceeds, if any, after reletting the Premises,
     after deducting Landlord's Reletting Expenses. "Reletting Expenses" is
     defined to include all expenses incurred by landlord in connection with
     reletting the Premises, including without limitation, all repossession
     costs, brokerage commissions, attorneys' fees, remodeling and repair costs,
     costs for removing and storing Tenant's property and equipment, and rent
     concessions granted by Landlord to any new Tenant, prorated over the life
     of the new lease.

     c. NONPAYMENT OF ADDITIONAL RENT. All costs which Tenant agrees to pay to
     Landlord pursuant to this Lease shall in the event of nonpayment be treated
     as if they

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     were payments of Rent, and Landlord shall have all the rights herein
     provided for in case of nonpayment of Rent.

     d. FAILURE TO REMOVE PROPERTY. If Tenant fails to remove any of its
     property from the Premises at Landlord's request following an uncured Event
     of Default, Landlord may, at its option, remove and store the property at
     Tenant's expense and risk.

21.  MORTGAGE SUBORDINATION AND ATTORNMENT. This Lease shall automatically be
     subordinate to any mortgage or deed of trust created by Landlord which is
     now existing or hereafter placed upon the Premises including any advances,
     interest, modifications, renewals, replacements or extensions ("LANDLORD'S
     MORTGAGE"), provided the holder of any Landlord's Mortgage or any person(s)
     acquiring the Premises at any sale or other proceeding under any such
     Landlord's Mortgage shall elect to continue this Lease in full force and
     effect. Tenant shall attorn to the holder of any Landlord's Mortgage or any
     person(s) acquiring the Premises at any sale or other proceeding under any
     Landlord's Mortgage provided such person(s) assume the obligations of
     Landlord under this Lease to the extent such obligations first arise after
     such acquisition. Tenant shall promptly and in no event later than fifteen
     (15) days execute, acknowledge and deliver documents which the holder of
     any Landlord's Mortgage may reasonably require as further evidence of this
     subordination and attornment including, without limitation, a
     subordination, non-disturbance and attornment agreement in the form
     attached hereto as Exhibit 21. Notwithstanding the foregoing, Tenant's
     obligations under this Section are conditioned on the holder of each of
     Landlord's Mortgage and each person acquiring the Premises at any sale or
     other proceeding under any such Landlord's Mortgage not disturbing Tenant's
     occupancy and other rights under this Lease, so long as no uncured Event of
     Default exists.

22.  NON-WAIVER. Landlord's waiver of any breach of any term contained in this
     Lease shall not be deemed to be a waiver of the same term for subsequent
     acts of Tenant. The acceptance by Landlord of Rent or other amounts due by
     Tenant hereunder shall not be deed to be a waiver of any breach by Tenant
     preceding such acceptance.

23.  HOLDOVER. If Tenant shall, without the written consent of Landlord, hold
     over after the expiration or termination of the Term, such tenancy shall be
     deemed to be on a month-to-month basis at a rental rate which is 150% of
     the Base Rent. All other terms of the Lease shall remain in effect.

24.  NOTICES. All notices under this Lease shall be in writing and effective (i)
     when delivered in person, (ii) three (3) days after being sent by
     registered or certified mail to Landlord or Tenant, as the case may be, at
     the Notice Addresses set forth in Section 1(g); or (iii) upon confirmed
     transmission by facsimile to such person at the facsimile numbers set forth
     in Section 1(g) or such other addresses/facsimile numbers as may from time
     to time be designated by such parties in writing.

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25.  COSTS AND ATTORNEYS' FEES. If Tenant or Landlord engage the services of an
     attorney to collect monies due or to bring any action for any relief
     against the other, declaratory or otherwise, arising out of this Lease,
     including any suit by Landlord for the recovery of Rent or other payments,
     or possession of the Premises, the losing party shall pay the prevailing
     party's attorneys' fees in such suit, at trial and on appeal.

26.  ESTOPPEL CERTIFICATES. Tenant shall, from time to time, upon written
     request of Landlord, execute, acknowledge and deliver to Landlord or its
     designee a written statement specifying the following, subject to any
     modifications necessary to make such statements true and complete: (i) the
     date the Lease term commenced and the date it expires; (ii) the amount of
     minimum monthly Rent and the date to which such Rent has been paid; (iii)
     that this Lease is in full force and effect and has not been assigned,
     modified, supplemented or amended in any way; (iv) that this Lease
     represents the entire agreement between the parties; (v) that all
     conditions under this Lease to be performed by Landlord have been
     satisfied; (vi) that there are not existing claims, defenses or offsets
     which the Tenant has against the enforcement of this Lease by Landlord;
     (vii) that no Rent has been paid more than one month in advance; (viii)
     that no security has been deposited with Landlord (or, if so, the amount
     thereof). Any such statement delivered pursuant to this Section may be
     relied upon by a prospective purchaser of Landlord's interest or assignee
     of any mortgage or new mortgagee of Landlord's interest in the Premises, if
     Tenant shall fail to respond within ten (10) days of receipt by Tenant of a
     written request by Landlord as herein provided, Tenant shall be deemed to
     have given such certificate as above provided without modification and
     shall be deemed to have admitted the accuracy of any information supplied
     by Landlord to a prospective purchaser or mortgages.

27.  TRANSFER OF LANDLORD'S INTERESTS. This Lease shall be assignable by
     Landlord without the consent of Tenant, in the event of any transfer or
     transfer of Landlord's interest in the Premises, other than a transfer for
     security purposes only, upon the assumption of this Lease by the
     transferee, Landlord shall be automatically relieved of obligations and
     liabilities accruing from and after the date of such transfer, except for
     any retained security deposit or prepaid rent, and Tenant shall attorn to
     the transferee.

28.  RIGHT TO PERFORM. If Tenant shall fail to timely pay any sum or perform any
     other act on its part to be performed hereunder, Landlord may make any such
     payment or perform any such other act on Tenant's part to be made or
     performed as provided in this Lease, Tenant shall, on demand, reimburse
     Landlord for its expenses incurred in making such payment or performance.
     Landlord shall (in addition to any other right or remedy of Landlord
     provided by law) have the same rights and remedies in the event of the
     nonpayment of sums due under this Section as in the case of default by
     Tenant in the payment of Rent.

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29.  HAZARDOUS MATERIAL. Tenant shall not cause or permit any Hazardous Material
     to be brought upon, kept, or used in or about, or disposed of on the
     Premises by Tenant, its agents, employees, contractors or invitees, except
     in strict compliance with all applicable federal, state and local laws,
     regulations, codes and ordinances (collectively, "Applicable Environmental
     Laws"). If Tenant breaches the obligations stated in the preceding
     sentence, then Tenant shall indemnify, defend and hold Landlord harmless
     from any and all claims, judgments, damages, penalties, fines, costs,
     liabilities or losses incurred or suffered by Landlord either during or
     after the Lease term. This indemnification by Tenant includes, without
     limitations, costs incurred in connection with any investigation of site
     conditions or any clean-up, remedial, removal or restoration work, whether
     or not required by any federal, state or local governmental agency or
     political subdivision, because of Hazardous Material present in the
     Premises, or in soil or ground water on or under the Premises. Tenant shall
     immediately notify Landlord of any inquiry, investigation or notice that
     Tenant may receive from any third party regarding the actual or suspected
     presence of Hazardous Material on the Premises.

     Without limiting the foregoing, if the presence of any "Hazardous Material"
     brought upon, kept or used in or about the Premises by Tenant, its agents,
     employees, contractors or invitees, results in any unlawful release of
     Hazardous Material on the Premises or any other property, Tenant shall
     promptly take all actions, at its sole expense, as are required under
     Applicable Environmental Laws, to the condition existing prior to the
     release of any such Hazardous Material.

     As used herein, the term "HAZARDOUS MATERIAL" means any hazardous,
     dangerous, toxic or harmful substance, material or waste including
     biomedical waste which is or becomes regulated by any local governmental
     authority, the Commonwealth of Massachusetts or the United States
     Government due to its potential harm to the health, safety or welfare of
     humans or the environment.

30.  QUIET ENJOYMENT. So long as Tenant pays the Rent and performs all of its
     obligations in this Lease, Tenant's possession of the Premises will not be
     disturbed by Landlord or anyone claiming by, through or under Landlord, or
     by the holders of any Landlord's Mortgage or any successor thereto.

31.  RIGHT OF ENTRY. Landlord and its agents, employees and contractors shall
     have the right to enter the Premises at reasonable times for inspection, to
     make repairs, alterations, and improvements, to show the Premises to
     prospective purchasers and, within six (6) months prior to the expiration
     of the Lease term, to show the Premises to prospective tenants.

32.  GENERAL.

                                       12
<PAGE>

     a. SUCCESSORS AND ASSIGNS. This Lease shall apply to and be binding upon
     Landlord and Tenant and their respective successors and assigns permitted
     hereunder. In no event shall Landlord ever have any liability under the
     Lease in excess of Landlord's equity in the Premises, and Tenant shall not
     seek damages or a judgment in excess of such amount of equity.

     b. BROKERS' FEES. Tenant represents and warrants to Landlord that it has
     not engaged any broker, finder or other person who would be entitled to any
     commission or fees for the negotiations, execution, or delivery of this
     Lease. Tenant shall indemnify and hold Landlord harmless against any loss,
     cost, liability or expense incurred by Landlord as a result of any claim
     asserted by any such broker, finder or other person on the basis of any
     arrangements or agreements made or alleged to have been made by or on
     behalf of Tenant. This subparagraph shall not apply to brokers' with whom
     Landlord has an express written brokerage agreement.

     c. ENTIRE AGREEMENT. This Lease contains all of the covenants and
     agreements between Landlord and Tenant relating to the Premises. No prior
     or contemporaneous agreements or understanding pertaining to the Lease
     shall be valid or of any force or effect and the covenants and agreements
     of this Lease shall not be altered, modified or added to except in writing
     signed by Landlord and Tenant.

     d. SEVERABILITY. Any provision of this Lease which shall prove to be
     invalid, void or illegal shall in no way affect, impair or invalidate any
     other provision of this Lease.

     e. FORCE MAJEURE. Time periods for either party's performance under any
     provisions of this Lease shall be extended for periods of time during which
     the party's performance is prevented due to circumstances beyond such
     party's control, including without limitation, fires, floods, earthquakes,
     lockouts, strikes, embargoes, governmental regulations, acts of God, public
     enemy, war or other strife.

     f. GOVERNING LAW. This Lease shall be governed by and construed in
     accordance with the laws of The Commonwealth of Massachusetts.

     g. AUTHORITY OF PARTIES. Any individual signing this Lease on behalf of any
     entity represents and warrants to the other that such individual has
     authority to do so and, upon such individual's execution, that this Lease
     shall be binding upon and enforceable against the party on behalf of whom
     such individual is signing.

                                       13
<PAGE>

         IN WITNESS WHEREOF, this Lease has been executed the date and year
first written above.

                               LANDLORD:

                               830 WINTER STREET LLC, a Delaware limited
                               liability company

                               By:  /s/ Kevin F. McLaughlin
                                    -------------------------------------------
                               Name:  Kevin F. McLaughlin
                               Title: Senior Vice President

                               TENANT:

                               PRAECIS PHARMACEUTICALS
                               INCORPORATED, a Delaware corporation

                               By:  /s/ Kevin F. McLaughlin
                                    -------------------------------------------
                               Name:  Kevin F. McLaughlin
                               Title: Senior Vice President







                                       14
<PAGE>

                                    EXHIBIT A

                          Legal Description of Premises

                                  See attached.















                                       15


<PAGE>


                                LEGAL DESCRIPTION

A certain parcel of land off Winter Street, in Waltham, Middlesex County,
Massachusetts, shown as Lot 9 on Land Court Plan No. 30618E, a copy of a
portion of which is filed with the Middlesex South Registry District of the
Land Court with Certificate of Title No. 214324 in Registration Book 1201,
Page 174.





                                       16

<PAGE>

                                  SCHEDULE 1(d)

                                      Rent

         Years 1-5                  $5,758,104/year; 479,842.00/month
         Years 6-10                 $6,107,080/year; 508,923.34/month
         Years 11-15                $6,456,056/year; 538,004.67/month

Landlord and Tenant acknowledge that a portion of the Rent is allocable to the
leasehold improvements as more particularly set forth on Exhibit A attached to
this Schedule 1(d); provided, however, that the foregoing shall in no way limit
or reduce Tenant's obligation to pay the full amount of Rent due and payable in
accordance with the terms of this Lease.






















                                       17
<PAGE>

                                    EXHIBIT A
                                       to
                                  SCHEDULE 1(d)

     Exhibit A to this Schedule 1(d) will be determined and attached to this
 Lease post-closing, subject to the reasonable approval of Landlord and Tenant.



















                                       18
<PAGE>

                                 SCHEDULE 15(a)

                                    Insurance

Business Interruption Insurance and any other insurance as may be reasonably
required by Landlord















                                       19
<PAGE>

                                   EXHIBIT 21

             Subordination, Nondisturbance and Attornment Agreement

                                  See attached.









                                       20


<PAGE>


             SUBORDINATION, NONDISTURBANCE AND ATTORNMENT AGREEMENT


         THIS AGREEMENT, made this ________ day of July, 2000, by and among
PRAECIS PHARMACEUTICALS INCORPORATED, a Delaware corporation with offices at
One Hampshire Street, Cambridge, Massachusetts 02139, (hereinafter called
"Tenant"), 830 Winter Street, a Delaware limited liability company having a
place of business at c/o PRAECIS PHARMACEUTICALS INCORPORATED, One Hampshire
Street, Cambridge, Massachusetts 02139 (hereinafter called "Landlord") and
ANGLO IRISH BANK CORPORATION PLC, having its principal place of business at
Stephen Court, 18-21 St. Stephen's Green, Dublin 2, Ireland (hereinafter
called "Mortgagee").

                              W I T N E S S E T H :

         WHEREAS, the Tenant has entered into a certain lease (the "Lease")
dated as of July 11, 2000 with the Landlord covering premises located at 830
Winter Street, Waltham, Middlesex County, Massachusetts (the "Premises"); and
more particularly described in Exhibit "A" attached hereto and incorporated
herein; and

         WHEREAS, the Mortgagee has agreed to make a mortgage loan (the
"Loan") to Landlord pursuant to the terms of the certain Acquisition and
Construction Loan Agreement (the "Loan Agreement"; which term includes all
modifications renewals, consolidations and extensions thereof) secured by,
among other security, a mortgage, security agreement and fixture filing (the
"Mortgage"; which term includes all modifications, renewals, replacements,
consolidations and extensions thereof) on the Premises from Landlord and a
certain Assignment of Leases and Rents from Landlord to Mortgagee (the
"Assignment"; which term includes all modifications, renewals, replacements,
consolidations and extensions thereof) pertaining to the Premises; and

         WHEREAS, Mortgagee has been requested by Tenant and Landlord to
enter into a nondisturbance agreement with Tenant.

         NOW, THEREFORE, in consideration of the premises and mutual
covenants hereinafter contained and other valuable consideration, the receipt
and sufficiency of which is hereby acknowledged, and in order to induce
Mortgagee to make the Loan to Landlord, the parties hereto covenant and agree
as follows:

         1. The Lease and any extensions, renewals, replacements or
modifications thereof, and all of the right, title and interest of the Tenant
in and to said Premises are and shall be subject and subordinate to the
Mortgage and to all of the terms and conditions contained therein (including,
without limitation, the casualty and condemnation provisions of the
Mortgage), and to any renewals, modifications, replacements, consolidations
and extensions thereof.


<PAGE>


         2. In the event of foreclosure of said Mortgage, or in the event
Mortgagee comes into possession, makes entry upon or acquires title to the
Premises as a result of the enforcement or foreclosure of the Mortgage or the
promissory note, or the Assignment or as a result of any other means,
Mortgagee agrees that the Lease shall not thereby be terminated and further
agrees that Tenant shall not be disturbed in its possession of the premises
demised under the Lease for any reason other than one which would entitle the
Landlord to terminate the Lease under its terms or would cause, without any
further action by such Landlord, the termination of the Lease or would
entitle such Landlord to dispossess the Tenant from such demised premises.

         3. Tenant agrees with Mortgagee that, if the interests of Landlord
in the Premises shall be transferred to and owned by Mortgagee by reason of
foreclosure or other proceedings brought by it, or by any other manner, or if
Mortgagee takes possession of or makes entry upon the Premises pursuant to
the Mortgage, the Assignment or any other document evidencing or securing the
Loan, Tenant shall be directly bound to Mortgagee under all of the terms,
covenants and conditions of the Lease for the balance of the term thereof
remaining and any extensions or renewals thereof which may be effected in
accordance with any option therefor in the Lease, with the same force and
effect as if Mortgagee were the Landlord under the Lease, and Tenant does
hereby attorn to Mortgagee as its Landlord, said attornment to be effective
and self-operative without the execution of any further instruments on the
part of any of the parties hereto immediately upon Mortgagee succeeding to
the interest of the Landlord in the Premises. Tenant agrees, however, upon
the election of and written demand by Mortgagee within twenty (20) days after
Mortgagee receives title to the Premises to execute an instrument in
confirmation of the foregoing provisions, satisfactory to Mortgagee, in which
Tenant shall acknowledge such attornment and shall set forth the terms and
conditions of its tenancy.

         4. Tenant agrees with Mortgagee that, if Mortgagee shall succeed to
the interest of Landlord under the Lease, Mortgagee shall not be (a) liable
for any act, waiver, representation (express or implied), or omission of any
prior landlord (the term "prior landlord" as used in this Section 4 includes,
without limitation, the Landlord) under the Lease or otherwise, or (b)
subject to any offsets, counterclaims or defenses which Tenant might have
against any prior landlord, or (c) bound by any rent, percentage rent or
additional rent or charges which Tenant might have paid for more than the
current month to any prior landlord, or (d) bound by any security deposit or
tax and/or insurance escrow which Tenant may have paid to any prior landlord,
unless such deposit and escrowed funds are in an escrow fund controlled by
Mortgagee, or (e) bound by any amendment or modification of the Lease or any
consent by any prior landlord under the Lease to any assignment or sublease
of the lessee's interest in the Lease made without Mortgagee's prior written
consent, or (f) bound by any provision in the Lease which obligates the
Landlord to erect or complete any building or to perform any construction
work or to make any improvements to the Premises or any parts thereof, or (g)
bound with respect to breaches other than those occurring during Mortgagee's
possession of the Premises or


                                       2


<PAGE>


ownership of the landlord's interest under the Lease. In addition, Tenant
agrees to look solely to the landlord's interest in the Premises for recovery
of any judgment from Mortgagee, it being specifically agreed that neither
Mortgagee nor anyone claiming under the Mortgagee shall ever be personally
liable for any such judgment. Tenant further agrees with Mortgagee that
Tenant will not voluntarily subordinate the Lease to any lien or encumbrance
without Mortgagee's prior written consent.

         5. Tenant hereby acknowledges that all of Landlord's right, title
and interest as lessor under the Lease (and in any guarantees of Tenant's
obligations thereunder) are being duly assigned to the Mortgagee pursuant to
the terms of the Mortgage and/or the Assignment and that pursuant to the
terms thereof all rental payments under the Lease shall continue to be paid
to Landlord in accordance with the terms of the Lease unless and until Tenant
is otherwise notified in writing by the Mortgagee. Upon receipt of any such
written notice from the Mortgagee, Tenant covenants and agrees to make
payment of all rental payments and other charges and payments then due or to
become due under the Lease directly to the Mortgagee or to the Mortgagee's
agent designated in such notice, whether or not the Mortgagee has made entry
or become mortgagee in possession pursuant to the Mortgage or the Assignment,
and to continue to do so until otherwise notified in writing by the
Mortgagee. Landlord hereby irrevocably directs and authorizes Tenant to make
rental payments directly to the Mortgagee following receipt of such notice,
and covenants and agrees that Tenant shall have the right to rely on such
notice without any obligation to inquire as to whether any default exists
under the Mortgage or the Assignment or the indebtedness secured thereby, and
notwithstanding any notice or claim of Landlord to the contrary, and that
Landlord shall have no right or claim against Tenant for or by reason of any
rental payments made by Tenant to the Mortgagee following receipt of such
notice. Tenant further acknowledges and agrees: (a) that under the provisions
of the Mortgage and/or Assignment, the Lease (and any guarantees thereof)
cannot be terminated (nor can Landlord accept any surrender of the Lease) or
modified in any of its terms, or consent be given to the waiver or release of
Tenant from the performance or observance of any obligation under the Lease
or to any assignments or subleases thereof, without the prior written consent
of the Mortgagee as set forth in the Loan Agreement, and without such consent
no rent may be collected or accepted by Landlord more than one month in
advance; and (b) that the interest of Landlord as lessor under the Lease (and
in any guarantees of Tenant's obligations thereunder) have been assigned to
the Mortgagee for the purposes specified in Mortgage and/or Assignment and
the Mortgagee assumes no duty, liability or obligation under the Lease,
except only under the circumstances, terms and conditions specifically set
forth in the Mortgage and/or the Assignment, copies of which are being
recorded concurrently herewith.

         6. Tenant, as lessee under the Lease, hereby covenants and agrees to
give the Mortgagee written notice properly specifying wherein the landlord
under the Lease has failed to perform any of the covenants or obligations of
the landlord under the Lease, simultaneously with the giving of any notice of
such default to the landlord under the provisions of the Lease. Tenant agrees
that the Mortgagee shall have the right, but not the obligation, within sixty
(60) days after receipt by the Mortgagee of such notice (or


                                       3


<PAGE>


within such additional time as is reasonably required to correct any such
default or is provided for in the Lease, whichever is longer) to correct or
remedy, or cause to be corrected or remedied, each such default before the
lessee under the Lease may take any action under the Lease by reason of such
default. Such notices to the Mortgagee shall be delivered in duplicate to:

                  Anglo Irish Bank Corporation PLC
                  Stephen Court
                  18-21 St. Stephen's Green
                  Dublin 2, Ireland
                  Attn:  Tom Browne, Director of Banking

                  with a copy of:

                  Anglo Irish Bank Corporation, PLC
                  84 State Street, 4th Floor
                  Boston, MA  02109
                  Attn:  David Drum, Executive Vice President

                  with a copy to:

                  Michael J. Haroz, Esq.
                  Goulston & Storrs, P.C.
                  400 Atlantic Avenue
                  Boston, MA 02110


or to such other address as the Mortgagee shall have designated to Tenant by
giving written notice to Tenant at One Hampshire Street, Cambridge,
Massachusetts 02139, or to such other address as may be designated by written
notice from Tenant to the Mortgagee. All written notices required or
permitted hereunder shall be sent by registered or certified mail, return
receipt requested, or by recognized overnight delivery service (such as
Federal Express) by telex or fax with confirmation in writing mailed
first-class, in all cases with postage and charges prepaid, and shall be
considered effective when received.

         7. This Agreement shall bind and inure to the benefit of the parties
hereto, their successors and assigns. Whenever a reference is made herein to
a requirement for Mortgagee's consent, such reference shall mean that
Mortgagee may give or withhold consent in its sole discretion. As used
herein, the term "Tenant" shall include the Tenant, its successors and
assigns, and the term "Landlord" shall include the Landlord and its
successors and assigns. The foregoing references to successors and assigns of
Tenant and Landlord is not intended to and does not constitute a consent by
Landlord or Mortgagee to any assignment or sublease by Tenant of its
interests under the Lease or any consent by Mortgagee to any assignment by
Landlord of its interests under the Lease. The words "foreclosure" and
"foreclosure sale" as used herein shall be deemed to include the acquisition
of Landlord's estate in the Premises by voluntary deed (or assignment) in
lieu of foreclosure, and the word "Mortgagee" shall include the Mortgagee
herein specifically


                                       4


<PAGE>


named and any of its successors and assigns, and anyone who shall have
succeeded to Landlord's interest in the Premises by, through or under
foreclosure of the Mortgage, including, without limitation, any purchaser of
the Premises through foreclosure or any successor or assign thereof.

         8. This Agreement shall not be modified or amended except in writing
signed by all parties hereto.

         9. The use of the neuter gender in this Agreement shall be deemed to
include any other gender, and words in the singular number shall be held to
include the plural, when the sense requires.

         IN WITNESS WHEREOF, the parties hereto have placed their hands and
seals, the day and year first above written.

WITNESS:                    TENANT:

                            PRAECIS PHARMACEUTICALS,
                            INCORPORATED
                            a Delaware corporation


______________________       By:  ________________________
                                  Name:
                                  Title:
                                  Hereunto duly authorized


WITNESS:                     830 Winter Street,
                             a Delaware limited liability company

                             By: PRAECIS PHARMACEUTICALS
                                 INCORPORATED,
                             a Delaware corporation,
                             its sole member


______________________       By:  ________________________
                                  Name:
                                  Title:
                                  Hereunto duly authorized


                                       5


<PAGE>


WITNESS:                      MORTGAGEE:

                              ANGLO IRISH BANK CORPORATION PLC


_______________________       By:  ________________________
                                   Name:
                                   Its:
                                   Hereunto duly authorized


                                       6


<PAGE>


                                 ACKNOWLEDGMENT


COMMONWEALTH OF MASSACHUSETTS)
                             ) ss.                       July ____, 2000
COUNTY OF SUFFOLK            )

Before me, a Notary Public in and for said County and State, personally
appeared _________ known to me to be the _______________ of PRAECIS
PHARMACEUTICALS INCORPORATED corporation which executed the foregoing
instrument, who acknowledged that the foregoing instrument was signed and
sealed for and on behalf of said corporation by authority of its Board of
Directors and that the same is his free act and deed as such ____________ and
the free act and deed of said corporation.

                                       -----------------------------
                                       Notary Public
                                       My Commission expires: _____



COMMONWEALTH OF MASSACHUSETTS)
                             ) ss.                 July ___, 2000
COUNTY OF SUFFOLK            )


Before me, a Notary Public in and for said County and State, personally
appeared ________ known to me to be the ______________ of PRAECIS
PHARMACEUTICALS INCORPORATED, the sole member of 830 Winter Street LLC, which
executed the foregoing instrument, who acknowledged that the foregoing
instrument was signed and sealed for and on behalf of said corporation on
behalf of said limited liability corporation by authority of its Board of
Directors and that the same is his free act and deed as such ___________ and
the free act and deed of said corporation and said limited liability
corporation.

                                        -----------------------------
                                        Notary Public
                                        My Commission expires: _____



                                       7


<PAGE>


COMMONWEALTH OF MASSACHUSETTS)
                             ) ss.                  ______________, 2000
COUNTY OF SUFFOLK            )


         Before me appeared______________________ to me known to be the
________ of Anglo Irish Bank Corporation PLC, the bank that executed the
annexed instrument, and acknowledged the said instrument to be his/her free
act and deed and the free act and deed of said bank, for the uses purposes
therein mentioned, and on oath stated that he/she was authorized to execute
said instrument on behalf of said bank.

                                      -----------------------------
                                      Notary Public
                                      My Commission expires: _____




                                       8


<PAGE>



                                    EXHIBIT A


                                LEGAL DESCRIPTION


                                [Attached behind]


<PAGE>

                                LEGAL DESCRIPTION

A certain parcel of land off Winter Street, in Waltham, Middlesex County,
Massachusetts, shown as Lot 9 on Land Court Plan No. 30618E, a copy of a
portion of which is filed with the Middlesex South Registry District of the
Land Court with Certificate of Title No. 214324 in Registration Book 1201,
Page 174.