Document:

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                                                                  Exhibit 10.5

                ENVIRONMENTAL COMPLIANCE AND INDEMNITY AGREEMENT

     This environmental compliance and indemnity agreement ("Environmental
Indemnity" or "Agreement") is given pursuant to the terms and conditions of the
agreement dated as of even date captioned "Acquisition and Construction Loan
Agreement" (the "Loan Agreement") by and among 830 WINTER STREET LLC, a Delaware
limited liability company having an address at c/o PRAECIS PHARMACEUTICALS
INCORPORATED, One Hampshire Street, Cambridge, Massachusetts 02139 (the
"Borrower"), PRAECIS PHARMACEUTICALS INCORPORATED, a Delaware corporation having
an address at One Hampshire Street, Cambridge, Massachusetts 02139 ("Guarantor")
and ANGLO IRISH BANK CORPORATION PLC ("Lender"; which term shall include any
other lenders that become "Lenders" in the future under the Loan Agreement)
having an address at Stephen Court, 18-21 St. Stephen's Green, Dublin 2,
Ireland.

     As used herein:

     A. The term "Environmental Legal Requirements" shall mean all applicable
past (which have current effect), present or future federal, state, county and
local laws, by-laws, rules, regulations, codes and ordinances, or any judicial
or administrative interpretations thereof, and the requirements of any
governmental agency or authority having or claiming jurisdiction with respect
thereto, including, without limitation, all orders, decrees, judgments, rulings,
requirements, directives or notices of violation, imposed through any public or
private enforcement proceedings, that create one or more duties, obligations,
responsibilities or liabilities with respect to:

     (i)  the regulation or protection of the environment;

     (ii) the health and safety of persons and property;

     (iii)any environmental pollution, impairment or disruption;

     (iv) any environmental permits (including, without limitation, the City of
          Waltham's Board of Health Recombinant DNA Permit related to the
          Property), licenses, emissions or affluent reduction plans and
          reporting requirements; and

     (v)  in each instance further including, without limitation, laws governing
          the existence, use, storage, treatment, discharge, release,
          containment, transportation, generation, manufacture, refinement,
          handling, production, disposal or management of any Hazardous
          Materials, or otherwise regulating or providing for the protection of
          the environment, and further including, without limitation, the
          Comprehensive Environmental Response Compensation and Liability Act
          (42 U.S.C.Section 9601 ET SEQ.), the Hazardous

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          Materials Transportation Act (49 U.S.C. Section 1801 ET SEQ.), the
          Public Health Service Act (42 U.S.C. Section 300(f) ET SEQ.), the
          Pollution Prevention Act (42 U.S.C. Section 13101 ET SEQ.), the
          Federal Insecticide, Fungicide and Rodenticide Act (7 U.S.C.
          Section 136 ET SEQ.), the Resource Conservation and Recovery Act
          (42 U.S.C. Section 6901 ET SEQ.), the Federal Clean Water Act (33
          U.S.C. Section 1251 ET SEQ.), the Federal Clean Air Act (42 U.S.C.
          Section 7401 ET SEQ.), and the Massachusetts Oil and Hazardous
          Materials Release Prevention and Response Act, M.G.L. c. 21E and
          the Massachusetts Hazardous Waste Management Act, M.G.L. c. 21C.

     B. The term "Hazardous Materials" shall mean and include asbestos,
flammable materials, explosives, radioactive or nuclear substances,
polychlorinated biphenyls, other carcinogens, oil and other petroleum products,
radon gas, urea formaldehyde, chemicals, gases, solvents, pollutants or
contaminants that could be a detriment or pose a danger to the environment or to
the health or safety of any person, and any other hazardous or toxic materials,
wastes and substances which are defined, determined or identified as such in any
past (which have current effect), present or future federal, state or local
laws, by-laws, rules, regulations, codes or ordinances or any judicial or
administrative interpretation thereof.

     C. The term "Property" shall mean property known as 830 Winter Street,
Waltham, Middlesex (Southern District) County, Massachusetts as further
described on EXHIBIT A which is annexed hereto and made apart hereof, together
with all appurtenant easement areas thereto and shall also include any other
tangible property in which Lender now or hereafter receives a mortgage or
security interest in connection with the Loan.

     D. The term "Surrounding Property" shall mean any property located within
one (1) mile of the perimeter of the Property.

     E. The term "Guarantor" shall mean PRAECIS PHARMACEUTICALS INCORPORATED, a
Delaware corporation.

     F. The term "Indemnitors" shall mean, collectively, the Borrower and the
Guarantor.

     G. The term "Indemnified Party" shall mean: (i) Lender; (ii) all those
claiming by, through or under Lender, including any other present or future
holder of the Loan or any portion thereof and any present or future owner of a
participation interest therein; (iii) any subsequent owner or tenant of all or
any portion of the Property following the exercise by Lender of the rights of
Lender under the Loan Agreement, the Security Deed, the Assignment of Leases and
Rents or the other Loan Documents including, but not limited to, a foreclosure
sale or deed in lieu thereof (provided, however, with respect to this clause
(iii), the representations, warranties, covenants and indemnities contained in
this Agreement shall only relate to environmental conditions arising out of
activities and conditions on or about the Property which occurred or existed

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prior to the Lender Acquisition Date (hereinafter defined) and which were caused
by Borrower on or after the date hereof); and (iv) as to each of the foregoing,
their respective affiliate, parent and subsidiary corporations, and, as
applicable, the respective officers, directors, stockholders, agents, employees,
accountants and attorneys of any one or more of them, and any person firm or
entity which controls, is controlled by, controlling, or under common control
with, any one or more of them. Notwithstanding the foregoing, it is agreed that
the term "Indemnified Party" shall not include (i) a party unaffiliated with
Lender who purchases the Property from Lender (or Lender's nominee) following
the expiration of thirteen (13) months from the date Lender (or Lender's
nominee) first acquires the Property at foreclosure sale, deed in lieu thereof,
or otherwise ("Lender Acquisition Date") or (ii) a party unaffiliated with
Lender who purchases the Property from Lender at foreclosure sale, or any party
unaffiliated with Lender to whom Lender sells, assigns, leases or otherwise
transfers the Property prior to the expiration of the thirteen (13) month period
following the Lender Acquisition Date, if Indemnitors shall have delivered to
Lender, or Lender shall have otherwise obtained prior to sale or other
disposition of the Property to any such party, a then current environmental site
assessment report (the "Report") prepared by a licensed site professional and
complying with the provisions of Section 4.1.1 hereof, which Report does not
disclose to the reasonable satisfaction of Lender the existence of any Hazardous
Materials condition in non-compliance, or the assertion by any environmental
agency or third party of any such condition in non-compliance, with
Environmental Legal Requirements applicable to the Property or the Surrounding
Property, resulting from Hazardous Materials either (x) originating, on, under,
or from the Property or (x) originating on, under, or from any Surrounding
Property and affecting the Property, and which has not been cured to the
reasonable satisfaction of Lender.

     H. The term "Environmental Enforcement Action" shall mean all actions,
orders, requirements or liens instituted, threatened in writing, required,
completed, imposed or placed by any governmental authority and all claims made
or threatened by any other person against or with respect to the Property or any
Surrounding Property, or any present or past owner or occupant thereof, arising
out of or in connection with any of the Environmental Legal Requirements, any
environmental condition, or the assessment, monitoring, clean-up, containment,
remediation or removal of, or damages caused or alleged to be caused by, any
Hazardous Materials (i) located on or under the Property or any Surrounding
Property, (ii) emanating from the Property or any Surrounding Property, or (iii)
generated, stored, transported, utilized, disposed, managed, or released by any
Indemnitor (whether or not on, under or from the Property or the Surrounding
Property).

     I. The terms "generated, "stored, "transported, "utilized," "disposed,"
"managed," "released" and "threat of release," and all conjugates thereof, shall
have the meanings and definitions set forth in the Environmental Legal
Requirements.

     J. The term "Environmental Report" shall mean the report identified on
EXHIBIT B which is annexed hereto and made a part hereof.

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     K. All other capitalized terms used in this Agreement which are not
otherwise specifically defined herein shall have the same meaning herein as in
the Loan Agreement.

     FOR VALUE RECEIVED, and to induce Lender to grant the Loan and extend
credit to Borrower as provided for in the Loan Agreement and the other Loan
Documents, Indemnitors hereby unconditionally agree as follows:

     1.   COMPLIANCE WITH ENVIRONMENTAL LEGAL REQUIREMENTS.

          1.1  COMPLIANCE. Until such time as all Obligations with respect to
          the Loan have been paid in full, Indemnitor guaranty and agree that
          Borrower, or the other Indemnitor on behalf of Borrower, shall comply
          with all Environmental Legal Requirements applicable to Borrower
          and/or the Property and that Borrower, or the other Indemnitor on
          behalf of Borrower, shall take all remedial action necessary to avoid
          any liability of Borrower, any Lender, or any subsequent owner of the
          Property, and to avoid the imposition of, or to discharge, any liens
          on the Property, as a result of any failure by Borrower to comply with
          Environmental Legal Requirements applicable to the Property.

          1.2  PROHIBITIONS. Without limitation upon the generality of
          foregoing, Indemnitors and each of them agree that they shall not:

          (a)  except in strict compliance with Environmental Legal
          Requirements, release or permit any release or threat of release of
          any Hazardous Materials on the Property;

          (b)  except in strict compliance with Environmental Legal
          Requirements, generate or permit any Hazardous Materials to be
          generated on the Property;

          (c)  except in strict compliance with all Environmental Legal
          Requirements, store, or utilize, or permit any Hazardous Materials to
          be stored or utilized on the Property;

          (d)  dispose of or permit any Hazardous Materials to be disposed of on
          the Property; and

          (e)  fail to operate, maintain, repair and use the Property in
          accordance with Environmental Legal Requirements; or

          f)   allow, permit or suffer any other person or entity to operate,
          maintain, repair and use the Property except in accordance with
          Environmental Legal Requirements.

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     2.   NOTICE OF CONDITIONS. Except as previously disclosed to Lender in the
Environmental Report, Indemnitors shall provide Lender with prompt written
notice, but in no event later than ten (10) Business Days after obtaining any
actual knowledge or actual notice thereof, of any of the following conditions:
(i) the presence, or any release or threat of release, of any Hazardous
Materials on, under or from the Property, or any Surrounding Property, whether
or not caused by any of Indemnitors; (ii) any Environmental Enforcement Action
instituted or threatened in writing; or (iii) any condition or occurrence on the
Property, or any Surrounding Property, that may constitute or threaten to cause
to occur a violation of any of the Environmental Legal Requirements with respect
to the Property.

     3.   INDEMNITORS' AGREEMENT TO TAKE REMEDIAL ACTIONS.

          3.1  REMEDIAL ACTIONS. Upon any of Indemnitors becoming aware of the
          violation of any Environmental Legal Requirement related to the
          Property, or the presence, or any release or any threat of release, of
          any Hazardous Materials on, under, or from the Property or any
          Surrounding Property, whether or not caused by any of Indemnitors,
          Indemnitors shall, subject to the rights to contest set forth in
          Section 6, immediately take all actions: (A) to cure or eliminate any
          such violation of any such Environmental Legal Requirement relating to
          the Property and, where applicable, to arrange for the assessment,
          monitoring, clean-up, containment, removal, remediation, or
          restoration of the Property and, (B) to the extent that the presence
          of any Hazardous Materials on the Surrounding Property originated on,
          under, or from the Property, or constitutes a danger to the Property,
          take such steps as (i) are required pursuant to any Environmental
          Legal Requirements or by any governmental authority to cure or
          eliminate violation(s) of any such Environmental Legal Requirement;
          and (ii) may otherwise be advisable and reasonably requested by Lender
          consistent with Environmental Legal Requirements.

          3.2  SECURITY FOR COSTS. If the potential costs associated with the
          actions required in Section 3.1, the release of any lien against the
          Property, and the release or other satisfaction of the liability, if
          any, of any of Indemnitors with respect to the Property arising under
          or related to any of the Environmental Legal Requirements or any
          Environmental Enforcement Action are determined by Lender, in good
          faith, to exceed $500,000, Lender shall have the right to require
          Indemnitors to provide, and Indemnitors shall provide, within thirty
          (30) days after written request therefor, a bond, letter of credit or
          other similar financial assurance, in form and substance satisfactory
          to Lender, in its good faith judgment, evidencing to Lender's
          reasonable satisfaction that the necessary financial resources will be
          unconditionally available to pay for all of the foregoing.

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          3.3  ENVIRONMENTAL ASSESSMENTS. Lender shall have the right to require
          Indemnitors, at their own cost and expense, to obtain a professional
          environmental assessment of the Property in accordance with Lender's
          then standard environmental assessment requirements and sufficient in
          scope to comply with the requirements of Section 4 of this Agreement,
          upon the occurrence of any one or more of the following events: (i) an
          Event of Default; or (ii) upon receipt of any notice of any of the
          conditions specified in Section 2 of this Agreement.

     4.   LENDER'S RIGHTS TO INSPECT THE PROPERTY AND TAKE REMEDIAL ACTIONS.

          4.1  LENDER' S RIGHTS. Lender may exercise its rights and remedies
          under this Section 4 only upon and following the existence of one or
          more of the following events or conditions: (i) the Loan becomes due
          and payable in full (but has not been paid) either at its stated
          maturity or upon acceleration based upon an Event of Default; (ii) an
          Indemnified Party has taken possession of all or some portion of the
          Property based upon an Event of Default; (iii) an Indemnified Party
          has acquired title to all or some portion of the Property by virtue of
          foreclosure or deed in lieu of foreclosure; or (iv) a claim has been
          asserted against an Indemnified Party for which indemnification is
          provided in this Agreement, but none of Indemnitors has undertaken and
          is continuing to pursue commercially reasonable efforts to remediate,
          defend and otherwise indemnify any such Indemnified Party and to
          provide such Indemnified Party with reasonable assurances of such
          Indemnitor's ability, financially and otherwise, to satisfy
          Indemnitor's obligations. In any such events Lender shall have the
          right, but not the obligation, through such representatives or
          independent contractors as it may designate, to enter upon the
          Property and to expend funds to:

          4.1.1 ASSESSMENTS. Cause one or more environmental assessments of the
          Property to be undertaken, if Lender in its reasonable discretion
          determines that such assessment is appropriate. Such environmental
          assessments may include, without limitation, (i) detailed visual
          inspections of the Property, including without limitation all storage
          areas, storage tanks, drains, drywells and leaching areas; (ii) the
          taking of soils and surface and sub-surface water samples; (iii) the
          performance of soils and ground water analysis; and (iv) the
          performance of such other investigations or analysis as are necessary
          or appropriate and consistent with sound professional environmental
          engineering practice in order for Lender to obtain a complete
          assessment of the compliance of the Property and the use thereof with
          all Environmental Legal Requirements and to make a determination as to
          whether or not there is any risk of contamination (x) to the Property
          resulting from Hazardous Materials originating on, under, or from any
          Surrounding Property or (y) to any

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          Surrounding Property resulting from Hazardous Materials originating
          on, under, or from the Property;

          4.1.2 CURE. Cure any breach of the representations, warranties,
          covenants and conditions made by or imposed upon Indemnitors under
          this Agreement including, without limitation, any violation by any of
          Indemnitors, or by the Property, or by any other occupant, prior
          occupant or prior owner thereof, of any of the Environmental Legal
          Requirements applicable to the Property; provided, and on the
          condition that, none of the Indemnitors has undertaken and is
          diligently pursuing to completion the curing of any such breach in a
          manner reasonably satisfactory to Lender;

          4.1.3 PREVENTION AND PRECAUTION. Take all actions as are necessary to
          (i) prevent the migration of Hazardous Materials on, under, or from
          the Property to any other property; (ii) clean-up, contain, remediate
          or remove any Hazardous Materials on, under, or from any other
          property which Hazardous Materials originated on, under, or from the
          Property; or (iii) prevent the migration of any Hazardous Materials
          on, under, or from any other property to the Property; provided, and
          on the condition that, none of the Indemnitors has undertaken and is
          diligently pursuing to completion the taking of all such necessary
          actions in a manner reasonably satisfactory to Lender;

          4.1.4 ENVIRONMENTAL ENFORCEMENT ACTIONS. Comply with, settle, or
          otherwise satisfy any Environmental Enforcement Action including, but
          not limited to, the payment of any funds or penalties imposed by any
          governmental authority and the payment of all amounts required to
          remove any lien or threat of lien on or affecting the Property, if,
          but only if, none of the Indemnitors has undertaken and is continuing
          to pursue efforts to comply with, settle, or otherwise satisfy any
          Environmental Enforcement Action in a manner reasonably satisfactory
          to Lender; and

          4.1.5 GENERAL. Comply with, settle, or otherwise satisfy any
          Environmental Legal Requirement and correct or abate any environmental
          condition on, or which threatens, the Property and which could cause
          damage or injury to the Property or the Surrounding Property or to any
          person, if, but only if, none of the Indemnitors has undertaken and is
          continuing to pursue efforts to comply with, settle, or otherwise
          satisfy any such Environmental Legal Requirement and correct or abate
          any such environmental condition in a manner reasonably satisfactory
          to Lender.

          4.2  RECOVERY OF COSTS. Any amounts paid or advanced by Lender
          and all costs and expenditures incurred in connection with any action
          taken pursuant to the terms of this Agreement, including but not
          limited to environmental consultants' and experts' fees and expenses,
          reasonable

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          attorneys' fees and expenses, court costs and all costs of assessment
          monitoring clean-up, containment, remediation, removal and
          restoration, with interest thereon at the Default Rate (as defined in
          the Note referred to in the Loan Agreement) shall be a demand
          obligation of Indemnitors to Lender and, to the extent not prohibited
          by law, (and so long as the Security Deed remains undischarged of
          record) shall be added to the Obligations secured by the Security Deed
          when paid by Lender and shall be secured by the lien of the Security
          Deed and the other Security Documents as fully and as effectively and
          with the same priority as every other obligation secured thereby.

          4.3  LENDER NOT RESPONSIBLE. The exercise by Lender of any one or more
          of the rights and remedies set forth in this Section 4 shall not
          operate or be deemed (a) to place upon Lender any responsibility for
          the operation, control, care, service, management, maintenance or
          repair of the Property or (b) make Lender the "owner" or "operator" of
          the Property or a "responsible party" within the meaning of any of the
          Environmental Legal Requirements.

          4.4  LENDER'S SUBROGATION. Furthermore, Lender, by making any such
          payment or incurring any such costs, shall be subrogated to all rights
          of each of Indemnitors or any other occupant of the Property to seek
          reimbursement from any other person including, without limitation, any
          predecessor, owner or occupant of the Property who may be a
          "responsible party" under any of the Environmental Legal Requirements
          in connection with the presence of Hazardous Materials on or under or
          which emanated from the Property.

          4.5  LENDER MAY STOP. Without limiting the generality of the other
          provisions of this Agreement, any partial exercise by Lender of any
          one or more of the rights and remedies set forth in this Section 4
          including, without limitation, any partial undertaking on the part of
          Lender to cure any failure by any of Indemnitors, or of the Property,
          or any other occupant, prior occupant or prior owner thereof, to
          comply with any of the Environmental Legal Requirements shall not
          obligate Lender to complete such actions taken or require Lender to
          expend further sums to cure such non-compliance.

     5.   INDEMNIFICATION. At all times, both before and after the repayment of
the Loan, Indemnitors hereby jointly and severally agree that they shall at
their sole cost and expense indemnify, defend, exonerate, protect and save
harmless each Indemnified Party against and from any and all damages, losses,
liabilities, obligations, penalties, claims, litigation, demands, defenses,
judgment, suits, proceedings, costs, disbursements or expenses of any kind or
nature whatsoever, including, without limitation, reasonable attorneys' and
experts' fees and disbursements, which may at any time be imposed upon,

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incurred by or asserted or awarded against any Indemnified Party and arising
from or out of:

          5.1  HAZARDOUS MATERIALS. Any Hazardous Materials on, in, under or
          affecting, all or any portion of the Property, or any Surrounding
          Property on or before the date hereof, or which may hereafter affect
          all or any portion of the Property or any Surrounding Property,
          resulting from Hazardous Materials either (i) originating, on, under,
          or from the Property or (ii) originating on, under, or from any
          Surrounding Property and affecting the Property, whenever discovered;

          5.2  ENVIRONMENTAL LEGAL REQUIREMENTS. The violation of or failure to
          comply with any Environmental Enforcement Action or any Environmental
          Legal Requirement by Borrower, or with respect to the Property or any
          Surrounding Property resulting from Hazardous Materials either (i)
          originating on, under, or from the Property or (ii) originating on,
          under, or from any Surrounding Property and affecting the Property,
          existing on or before the date hereof or which may so exist in the
          future, whenever discovered;

          5.3  BREACH OF WARRANTY, REPRESENTATION OR COVENANT. Any breach of
          warranty or representation or covenant made by any Indemnitor under or
          pursuant to this Agreement; and

          5.4  GENERAL. The enforcement of this Agreement or the assertion by
          any Indemnitor of any defense to the obligations of an Indemnitor
          hereunder, whether any of such matters arise before or after
          foreclosure of the Security Deed or other taking of title to or
          possession of all or any portion of the Property by Lender or any
          other Indemnified Party, and specifically including therein, without
          limitation, the following: (i) costs incurred for any of the matters
          set forth in Section 4 of this Agreement; and (ii) costs and expenses
          incurred in ascertaining the existence or extent of any asserted
          violation of any Environmental Legal Requirements relating to the
          Property and any remedial action taken on account thereof including,
          without limitation, the costs, fees and expenses of engineers,
          geologists, chemists, other scientists, attorneys, surveyors, and
          other professionals, or testing and analyses performed in connection
          therewith.

          5.5  EXCLUSION. Notwithstanding the foregoing, it is understood and
          agreed that no Indemnified Party shall be indemnified hereunder for
          any loss (a) to the extent first caused by acts or omissions of an
          Indemnified Party subsequent to the Lender Acquisition Date or the
          date Lender takes possession of the Property pursuant to its remedial
          rights under the Loan, or (b) after repayment of the Loan and the
          satisfaction of all conditions set

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          forth in Section 20 with respect to the termination of the continuing
          liability of Indemnitors under this Agreement.

     6.   RIGHT TO CONTEST. Borrower, or any other Indemnitor, may contest in
good faith any claim, demand, levy or assessment under any Environmental Legal
Requirements, including, but not limited to, any claim with respect to Hazardous
Materials, by any person or entity if:

          6.1  MATERIAL QUESTION IN GOOD FAITH. The contest is based upon a
          material question of law or fact raised by Borrower or such other
          Indemnitor in good faith;

          6.2  DILIGENT PURSUIT. Borrower or such other Indemnitor properly
          commences and thereafter diligently pursues the contest;

          6.3  NO IMPAIRMENT. The contest will not materially impair the taking
          of any required remedial action with respect to such claim, demand,
          levy or assessment;

          6.4  ADEQUATE RESOURCES. Borrower, or such other Indemnitor,
          demonstrates to Lender's reasonable satisfaction that Borrower, or
          such other Indemnitor, has the financial capability to undertake and
          pay for such contest and any remedial action then or thereafter
          necessary; and

          6.5  NO EOD. No Event of Default exists under the Loan Documents or,
          in Lender's reasonable judgment, no such Event of Default is likely to
          be precipitated by such claim, demand, levy or assessment.

     7.   WAIVERS. Indemnitors each hereby waive and relinquish to the fullest
extent now or hereafter not prohibited by applicable law:

          7.1  SURETYSHIP DEFENSES. All suretyship defenses and defenses in the
          nature thereof;

          7.2  MARSHALING. Any right or claim of right to cause a marshaling of
          Borrower's assets or of any Collateral or to cause Lender to proceed
          against any of the Collateral for the Loan before proceeding under
          this Agreement against any Indemnitor, or to require Lender to proceed
          against Indemnitors in any particular order;

          7.3  CONTRIBUTION. Until the full and Non-Contestable Payment and
          satisfaction of all Obligations, all rights and remedies against any
          other Indemnitor, including, but not limited to, any rights of
          subrogation, contribution, reimbursement, exoneration or
          indemnification pursuant to any express or implied agreement, now or
          hereafter accorded by applicable

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          law to indemnitors, guarantors, sureties or accommodation parties.
          Provided, however, unless Lender otherwise expressly agrees in
          writing, such waiver by any particular Indemnitor shall not be
          effective to the extent that by virtue thereof such Indemnitor's
          liability under this Indemnity Agreement or under any other Loan
          Document is rendered invalid, voidable, or unenforceable under any
          applicable state or federal law dealing with the recovery or avoidance
          of so-called preferences or fraudulent transfers or conveyances or
          otherwise;

          7.4  NOTICE. Notice of the acceptance hereof, presentment, demand for
          payment, protest, notice of protest, or any and all notice of
          nonpayment, nonperformance, nonobservance or default or other proof or
          notice of demand whereby to charge Indemnitors therefor;

          7.5  STATUTE OF LIMITATIONS. The pleading of any Statute of
          Limitations as a defense to such Indemnitor's obligations hereunder;

          7.6. JURY TRIAL. The right to a trial by jury in any matter related to
          this Environmental Indemnity; and

          EACH INDEMNITOR AND LENDER HEREBY KNOWINGLY, VOLUNTARILY AND
          INTENTIONALLY WAIVES THE RIGHT TO A TRIAL BY JURY IN RESPECT OF ANY
          LITIGATION BASED HEREON, ARISING OUT OF, UNDER OR IN CONNECTION WITH
          THIS AGREEMENT OR ANY OTHER LOAN DOCUMENTS CONTEMPLATED TO BE EXECUTED
          IN CONNECTION HEREWITH OR ANY COURSE OF CONDUCT, COURSE OF DEALINGS,
          STATEMENTS (WHETHER VERBAL OR WRITTEN) OR ACTIONS OF ANY PARTIES; THIS
          WAIVER BEING A MATERIAL INDUCEMENT FOR LENDER TO ACCEPT THIS AGREEMENT
          AND TO MAKE THE LOAN.

     8.   CUMULATIVE RIGHTS. Lender's rights under this Agreement shall be in
addition to and not in limitation of all of the rights and remedies of Lender
under the other Loan Documents. All rights and remedies of Lender shall be
cumulative and may be exercised in such manner and combination as Lender may
determine.

     9.   NO IMPAIRMENT. The liability of Indemnitors hereunder shall in no way
be limited or impaired by, and each Indemnitor hereby assents to and agrees to
be bound by, any amendment or modification of the provisions of the Loan
Documents to or with Lender and other Lenders by Borrower or any Indemnitor or
any person who succeeds Borrower as owner of the Property. In addition, the
liability of Indemnitors under this Agreement shall in no way be limited or
impaired by:

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          9.1  EXTENSIONS. Any extensions of time for performance required by
          any of the Loan Documents;

          9.2  AMENDMENTS. Any amendment to or modification of any of the Loan
          Documents;

          9.3  TRANSFER. Any sale or assignment of or participation in the Loan
          or any portion thereof, or any sale, assignment or foreclosure of the
          Note or Security Deed or any sale or other transfer of all or part of
          the Property, except as such liability is limited by the preceding or
          succeeding provisions of this Agreement;

          9.4  EXCULPATORY LANGUAGE. Any exculpatory, or nonrecourse, or limited
          recourse, provision in any of the Loan Documents limiting Lender's
          recourse to the Property encumbered by the Security Deed or to any
          other property or limiting Lender's rights to a deficiency judgment
          against Borrower or any other party;

          9.5  INACCURACIES. The accuracy or inaccuracy of any of the
          representations or warranties made by or on behalf of any Indemnitor
          under the Loan Documents or otherwise;

          9.6  RELEASE. The release of any Indemnitor, or of any other person or
          entity from performance or observance of any of the agreements,
          covenants, terms of conditions contained in any of the Loan Documents
          by operation of law, Lender's voluntary act, or otherwise;

          9.7  BANKRUPTCY OR REORGANIZATION. The filing of any bankruptcy or
          reorganization proceeding by or against any Indemnitor, any general
          partner, member or owner of any Indemnitor, Borrower, or any
          subsequent owner of the Property;

          9.8  SUBSTITUTION. The release or substitution in whole or part of any
          collateral or security for the Loan;

          9.9  FAILURE TO PERFECT. Lender's failure to record the Security Deed
          or file any UCC financing statements (or Lender's improper recording
          or filing of any thereof) or to otherwise perfect, protect, secure, or
          insure any security interest or lien given as security for the Loan;

          9.10 RELEASE OF PARTIES. The release of any one or more of Indemnitors
          or any other party now or hereafter liable upon or in respect of this
          Agreement or the Loan; or

                                      -12-
<PAGE>

          9.11 INVALIDITY. The invalidity or unenforceability of all or any
          portions of any of the Loan Documents as to any Indemnitor or to any
          other person or entity.

Any of the foregoing may be accomplished with or without notice to Borrower or
any Indemnitor or with or without consideration.

     10.  DELAY NOT WAIVER. No delay on Lender's part in exercising any right,
power or privilege hereunder or under any of the Loan Documents shall operate as
a waiver of any such privilege, power or right. No waiver by Lender in any
instance shall constitute a waiver in any other instance.

     11.  WARRANTIES AND REPRESENTATIONS. The Indemnitors each represent and
warrant to each Indemnified Party, the same to be true as of the date hereof, as
of the date of any request for or the making of any Advances under the Loan and
throughout the period that any of the Loan Documents shall remain in force and
effect, that except as otherwise disclosed in the Environmental Report:

          11.1 NO HAZARDOUS MATERIALS AT PROPERTY. To the best of their
          knowledge and belief, after due inquiry, except as previously
          disclosed to Lender in the Environmental Report or as permitted to
          occur in strict compliance with Environmental Legal Requirements, no
          Hazardous Materials have been or are currently generated, stored,
          transported, utilized, disposed of, managed, released or located on,
          under or from the Property, whether or not in reportable quantities,
          or in any manner introduced onto the Property including without
          limitation any drainage, septic, sewage or other waste disposal
          systems servicing the Property;

          11.2 NO VIOLATIONS CLAIMED RE PROPERTY OR INDEMNITORS. None of the
          Indemnitors has received any notice from the Massachusetts Department
          of Environmental Protection, the United States Environmental
          Protection Agency or any other governmental authority claiming that
          (i) the Property or any use thereof violates any of the Environmental
          Legal Requirements or (ii) any of the Indemnitors or any of their
          respective employees or agents have violated any of the Environmental
          Legal Requirements with respect to the Property or any Surrounding
          Property;

          11.3 NO LIABILITY TO GOVERNMENTAL AUTHORITIES. None of the Indemnitors
          has incurred any liability to the Commonwealth of Massachusetts or the
          City of Waltham or of any other governmental authority under any of
          the Environmental Legal Requirements;

          11.4 NO LIEN ON PROPERTY. No lien against the Property has arisen
          under or related to any of the Environmental Legal Requirements;

                                      -13-
<PAGE>

          11.5 NO ENFORCEMENT ACTIONS. There are no Environmental Enforcement
          Actions pending, or to the best of the Indemnitors' information,
          knowledge and belief after due inquiry, threatened in writing with
          respect to the Property.

          11.6 NO KNOWLEDGE OF HAZARDOUS MATERIALS AT SURROUNDING PROPERTY.
          Except as previously disclosed to Lender in the Environmental Report
          or as otherwise permitted to occur in strict compliance with
          Environmental Legal Requirements, none of the Indemnitors has any
          knowledge that any Hazardous Materials has been or is currently
          generated, stored, transported, utilized, disposed of, managed,
          released or located on, under or from the Surrounding Property in
          violation of or allegedly in violation of any of the Environmental
          Legal Requirements with respect to the Property;

          11.7 NO KNOWLEDGE OF VIOLATIONS RE SURROUNDING PROPERTY. Except as
          previously disclosed to Lender in the Environmental Report, none of
          the Indemnitors has any knowledge of any action or order instituted or
          threatened in writing by any person or governmental authority arising
          out of or in connection with the Environmental Legal Requirements
          involving the assessment, monitoring, cleanup, containment,
          remediation or removal of or damages caused or alleged to be caused by
          any Hazardous Materials generated, stored, transported, utilized,
          disposed of, managed, released or located on, under or from any
          Surrounding Property;

          11.8 NO UNDERGROUND STORAGE TANKS. There are no underground storage
          tanks on or under the Property;

          11.9 [Intentionally Deleted];

          11.10 VALID-AND BINDING. This Agreement constitutes the legal, valid
          and binding obligation of each of the Indemnitors in accordance with
          the respective terms hereof;

          11.11 ENTITY MATTERS. That each Indemnitor (other than one who is a
          natural person) is a duly organized validly existing entity in good
          standing under the laws of its organization and has all requisite
          power and authority to conduct its business and to own its properties
          as now conducted or owned;

          11.12 NO VIOLATIONS. The performance of the obligations evidenced
          hereby will not constitute a violation of any law, order, regulation,
          contract, organizational document or agreement to which the
          Indemnitors or any of them is a party or by which any one or more of
          them or their property is or may be bound;

                                      -14-
<PAGE>

          11.13 NO LITIGATION. There is no material litigation or administrative
          proceeding now pending or threatened in writing against the
          Indemnitors or any of them which if adversely decided could materially
          impair the ability of any one or more of the Indemnitors to pay or
          perform their respective obligations hereunder; and

          11.14 MATERIAL ECONOMIC BENEFIT. The granting of the Loan to Borrower
          will constitute a material economic benefit to each Indemnitor.

     12.  MULTIPLE COUNTERPARTS. This Agreement may be executed in one or more
counterparts, each of which shall be deemed an original. Each of the
counterparts shall constitute but one in the same instrument and shall be
binding upon each of the parties individually as fully and completely as if all
had signed but one instrument so that the joint and several liability of each of
the Indemnitors hereunder shall be unaffected by the failure of any of the
undersigned to execute any or all of said counterparts.

     13.  NOTICES. Any notices given pursuant to this Agreement shall be
sufficient only if given in the manner provided for in the Loan Agreement.

     14.  NO ORAL CHANGE. No provision of this Agreement may be changed, waived,
discharged, or terminated orally by telephone or by any other means except by an
instrument in writing signed by the party against whom enforcement of the
change, waiver or discharge or termination is sought.

     15.  PARTIES BOUND; BENEFIT. Except as specifically provided otherwise
herein, this Agreement shall be binding upon the Indemnitors and their
respective successors, assigns and shall be for the benefit of Lender, and of
any subsequent holder of the Loan and of any owner of a participation interest
therein. In the event the Loan is sold or transferred, then the liability of the
Indemnitors to Lender shall then be in favor of both the Lender originally named
herein and each subsequent holder of the Loan and any of interest therein. In
addition, Lender shall have the right to assign the benefits of this Agreement
to any purchaser of the Property from Lender or an affiliate of Lender in
connection with any exercise by Lender of its remedial rights under the Loan
provided, and on the condition that, any such purchaser of the Property is an
"Indemnified Party," as such term is defined in Paragraph F of the Recitals to
this Agreement.

     16.  JOINT AND SEVERAL. The obligations of each of the Indemnitors and
their respective successors, assigns, heirs and personal representatives shall
be joint and several.

     17.  PARTIAL INVALIDITY. Each of the provisions hereof shall be enforceable
against each Indemnitor to the fullest extent now or hereafter permitted by law.
The invalidity or unenforceability of any provision hereof shall not limit the
validity or enforceability of each other provision hereof.

                                      -15-
<PAGE>

     18.  GOVERNING LAW AND CONSENT TO JURISDICTION. This Agreement and the
rights and obligations of the parties hereunder shall in all respects be
governed by and construed and enforced in accordance with the laws of the
Commonwealth of Massachusetts, without giving effect to Massachusetts principles
of conflicts of law, and, insofar as Environmental Legal Requirements and other
Environmental Legal Requirements are concerned, in accordance with applicable
federal law and Massachusetts law as well. The parties further agree that Lender
may enforce its rights under this Agreement and the other Loan Documents
including, but not limited to, the rights to sue any Indemnitor in accordance
with applicable law. The Indemnitors hereby irrevocably submit to the
nonexclusive jurisdiction of any Massachusetts State or any Federal Court
sitting in Massachusetts over any suit, action or proceeding arising out of or
relating to this Agreement and the Indemnitors hereby agree and consent that in
addition to any methods of service of process provided for under applicable law,
all service of process in any such suit, action or proceeding in any
Massachusetts State or Federal Court sitting in Massachusetts may be made by
certified or registered mail, return receipt requested, directed to the
Indemnitors at the address indicated in Section 13 above and service so made
shall be completed five (5) days after the same shall have been so mailed.

     19. SURVIVAL. Subject to the limitations set forth in Paragraph G of the
Recitals above, and to the provisions of Section 20 below, the provisions of
this Agreement shall continue in effect and shall survive (among other events)
any payment and satisfaction of the Loan and the Obligations, any termination or
discharge of the Security Deed, foreclosure, a deed-in lieu transaction, or
release of any Collateral.

     20.  RIGHT OF INDEMNITORS TO TERMINATE OBLIGATIONS. The Indemnitors shall
have the right to terminate their continuing liability hereunder upon
fulfillment of each of the following conditions to the reasonable satisfaction
of Lender:

               (i) Lender shall have received Non-Contestable Payment in full of
          all Obligations, including but not limited to repayment in full of the
          Loan, but excluding any Obligations which might arise in the future
          under the provisions of this Agreement. The term "Non-Contestable"
          shall mean the receipt of payment of the Loan or other satisfaction of
          Obligations and the expiration of all periods of time within which a
          claim for the recovery of a preferential payment, or fraudulent
          conveyance, or fraudulent transfer, in respect of payments received by
          Lender as to the Obligations could be filed or asserted with (A) no
          such claim having been filed or asserted, or (B) if so filed or
          asserted, the final, non-appealable decision of a court of competent
          jurisdiction denying the claim or assertion.

               (ii) The Indemnitors shall have delivered to Lender a report
          complying with the provisions of Sections 3 and 4 hereof and such
          report does not disclose the existence of any violation of
          Environmental Legal

                                      -16-
<PAGE>

          Requirements applicable to the Property, or the lawful assertion by
          any governmental agency or third party of any violation which has not
          been cured of any Environmental Legal Requirements, on or with respect
          to the Property, which report shall be dated, or last updated, to a
          date which is not earlier than the date on which the Security Deed was
          discharged or released of record.

               (iii) No Environmental Enforcement Action shall be pending or
          threatened in writing with respect to the Property.

               (iv) The Loan has been repaid without Lender or any affiliate
          thereof ever having taken actual or constructive possession of any of
          the Property, through either: (A) the appointment of a receiver, or
          (B) any other exercise of Lender's rights and remedies following an
          Event of Default under any of the Loan Documents.

               (v) No change shall have occurred in applicable Environmental
          Legal Requirements or in any official interpretation thereof
          (including governmental, judicial or administrative) which would
          result in any Indemnified Party being held responsible for causing the
          Property not to be in compliance with Environmental Legal
          Requirements.

               (vi) At least thirteen (13) months have passed since the date the
          principal, interest and fees due Lender in respect of the Loan were
          paid (without regard to the passage of time required to establish
          Non-Contestable Payment) and no claim has been asserted for which any
          indemnification is provided for in this Agreement.

     Such termination of Indemnitors' liability hereunder shall become effective
only upon the delivery by Lender to the Indemnitors of a specific written
acknowledgment of the satisfaction of the foregoing conditions and the
termination of such obligations, which Lender agrees to provide unless Lender
makes the good faith reasonable determination that the conditions to such
termination have not been satisfied.

                                      -17-
<PAGE>

       Witness the execution and delivery hereof as an instrument under seal as
of the __ day of July, 2000.

                                       INDEMNITORS:

Attest:                                PRAECIS PHARMACEUTICALS
                                       INCORPORATED,
                                       a Delaware corporation

/s/ Mary E. DeLena
-------------------------              By: /s/ Kevin F. McLaughlin
                                          -------------------------------------

                                          Name: Kevin F. McLaughlin
                                               --------------------------------
                                          Title: Senior Vice President
                                                -------------------------------
                                          Hereunto duly authorized

Attest:                                830 WINTER STREET LLC,
                                       a Delaware limited liability company

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

/s/ Mary E. DeLena
-------------------------              By: /s/ Kevin F. McLaughlin
                                          -------------------------------------
                                          Name: Kevin F. McLaughlin
                                               --------------------------------
                                          Title: Senior Vice President
                                                -------------------------------
                                          Hereunto duly authorized

                                      -18-

<PAGE>

                                   EXHIBIT A

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.

Together with the benefit of rights reserved in Easement dated July 14, 1997,
filed as Document No. 1036276, and recorded in Book 27478, Page 136; as
affected by Utility Easement from owners of Lots 2 and 3 on Land Court Plan #
30618C and Lots 2, 3, B and C on Plan #669 of 1997, to Boston Edison Company
and New England Telephone and Telegraph Company, d/b/a Bell Atlantic, dated
August 27, 1998, filed as Document No. 1078157; as further affected by
Reciprocal Access and Utility Easement dated March 25, 1999, filed as
Document No. 1101665 and recorded March 26, 1999, as Instrument No. 503.

Together with the benefit of grant and reservation recited in Reciprocal Access
and Utility Easement with the owner of Lots 5 and 6 on Land Court Plan No.
30618D dated March 31, 1998, filed as Document No. 1061070, and recorded in Book
28405, page 421, affecting areas shown as "Reserved Easement Area" on a plan
entitled "Easement Plan of Land in Waltham, Massachusetts", dated March 30,
1998, recorded therewith; as affected by First Amendment to Reciprocal Access
and Utility Easement and to Reciprocal Easement Agreement dated September 10,
1998, filed as Document No. 1079645, and recorded in Book 29108, Page 346; as
further affected by Reciprocal Access and Utility Easement dated March 25, 1999,
filed as Document No. 1101665 and recorded March 26, 1999, as Instrument No.
503, and by Reciprocal Easement Agreement dated March 25, 1999, filed as
Document No. 1101666.

Together with the benefit of Reciprocal Easement Agreement with the owner of
Lots 5 and 6 on Land Court Plan No. 30618D dated March 31, 1998, filed as
Document No. 1061071, and recorded in Book 28405, Page 443, affecting areas
shown on a plan entitled "Easement Plan of Land in Waltham, Massachusetts",
dated March 30, 1998, recording therewith; as affected by First Amendment to
Reciprocal Access and Utility Easement and to Reciprocal Easement Agreement
dated September 10, 1998, filed as Document No. 1079645, and recorded in Book
29108, Page 346; as further affected by Reciprocal Easement Agreement dated
March 25, 1999, filed as Document No. 1101666.

Together with the benefit of Reciprocal Easement Agreement with the owner of Lot
8 on Land Court Plan No. 30618E dated March 10, 1999, filed as Document No.
1099963.

Together with the benefit of Reciprocal Access and Utility Easement Agreement
with the owner of Lot 8 on Land Court Plan No. 30618E dated March 10, 1999,
filed as Document No. 1099964, and recorded March 10, 1999 as Instrument No.
1121; as further affected by Reciprocal Access and Utility Easement dated
March 25, 1999, filed as Document No. 1101665 and recorded March 26, 1999, as
Instrument No. 503.

Together with the benefit of reciprocal Access and Utility Easement, dated
March 25, 1999, filed as Document No. 1101665.

Together with the benefit of Reciprocal Easement Agreement dated March 25, 1999,
filed as Document No. 1101666.

Together with the benefit of Mutual Covenants Agreement dated March 25, 1999,
filed as Document No. 1101667.

Together with the benefit of Landscape License Agreement dated March 25, 1999,
filed as Document No. 1101668.

<PAGE>

                                    EXHIBIT B

Phase I Environmental Assessment of the Vacant Land Parcel - 830 Winter Street,
Waltham Woods, Waltham, Massachusetts, dated February 1999, prepared by ENVIRON
International Corporation.

Updated Phase I Environmental Assessment of 830 Winter Street, Waltham Woods,
Waltham, Massachusetts, dated January 2000, prepared by ENVIRON International
Corporation.<PAGE>

                                                                   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

<PAGE>

     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

                                       3
<PAGE>

     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

                                       4
<PAGE>

     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.

                                       5
<PAGE>

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.

                                       6
<PAGE>

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.

                                       7
<PAGE>

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

                                       8
<PAGE>

     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

                                       9
<PAGE>

     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.

                                       10
<PAGE>

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.

                                       11
<PAGE>

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.

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