
<PAGE>
 
                                                        Exhibit 10.42


                     ENVIRONMENTAL INDEMNITY AND AGREEMENT
                     -------------------------------------

                   ENVIRONMENTAL INDEMNITY AND AGREEMENT (this "Agree-
                                                                ------
         ment"), dated as of November __, 1997, made by BOSTON PROPER-
         ----
         TIES LIMITED PARTNERSHIP ("Mortgagor"), a Delaware limited
                                    ---------
         partnership, having an address at 599 Lexington Avenue, New
         York, New York 10022, in favor of JOHN HANCOCK MUTUAL LIFE IN-
         SURANCE COMPANY ("Mortgagee"), a Massachusetts corporation,
                           ---------
         having its principal office at John Hancock Place, Post Office
         Box 111, Boston, Massachusetts 02117, Attention:  Real Estate
         Investment Group, T-52.


                                   WITNESSETH:
                                   ----------
                   WHEREAS, Mortgagor is, on the date hereof, (a) the
         owner of fee title to a parcel of land and the improvements
         thereon known as 875 Third Avenue, New York, New York and the
         easements appurtenant thereto (collectively, the "Original Fee
                                                           ------------
         Parcel"), all as more particularly described in Exhibit A-1
         ------                                          -----------
         hereto, and (b) the owner of fee title to the Corner Parcel and
         all Improvements thereon (as such terms are defined in the
         Mortgage, as hereinafter defined), which Corner Parcel is con-
         tiguous to the Original Fee Parcel and is more particularly
         described on Exhibit A-2 hereto (the Original Fee Parcel and
                      ----------- 
         the Corner Parcel are collectively described on Exhibit A-3
                                                         -----------
         hereto);

                   WHEREAS, Mortgagee is on the date hereof the owner
         and holder of certain mortgages which were consolidated, ex-
         tended and modified pursuant to that certain Consolidation,
         Extension and Modification Agreement (the "Consolidation Agree-
                                                    --------------------
         ment"), dated as of May 11, 1988, between Kenvic Associates, a
         ----
         New York general partnership ("Original Mortgagor") and Mort-
                                        ------------------
         gagee, recorded on May 18, 1988 in the Office of the City Reg-
         ister, New York County (the "Register's Office") in Reel 1403,
                                      -----------------
         Page 1793, so as to form a single first lien on the Mortgaged
         Premises securing the original principal amount of
         $180,000,000, and were further modified (i) pursuant to that
         certain Modification Agreement, dated as of May 30, 1990, be-
         tween Original Mortgagor and Mortgagee, recorded on June 28,
         1990 in the Register's Office in Reel 1705, Page 1760 (the
         "1990 Modification Agreement"), (ii) pursuant to that certain
          ---------------------------
         Note and Mortgage Modification Agreement, dated as of July 23,
         1992, between Original Mortgagor and Mortgagee, recorded on
         July 30, 1992 in the Register's Office in Reel 1892, Page 434
         (the "1992 Modification Agreement"), (iii) pursuant to that
               ---------------------------
         certain Note and Mortgage Modification and Spreader Agreement,
         dated as of December 29, 1995, between Original Mortgagor and<PAGE>
<PAGE>
 
         Mortgagee, recorded on January 5, 1996 in the Register's Office
         in Reel 2279, Page 443 (the "1995 Modification Agreement"); and
                                      ---------------------------
         (iv) pursuant to that certain Note and Mortgage Modification
         and Spreader Agreement, dated as of the date hereof, between
         Mortgagor and Mortgagee, intended to be recorded in the
         Register's Office following the execution thereof (the "1997
         Modification Agreement"; the Consolidation Agreement as amended
         ----------------------
         by the 1990 Modification Agreement, the 1992 Modification
         Agreement, the 1995 Modification Agreement and the 1997 Modifi-
         cation Agreement is hereinafter referred to as the "Mortgage";
                                                             --------
         any capitalized terms used herein and not defined herein shall
         have the respective meanings assigned to them in the Mortgage);

                   WHEREAS, as of the date hereof, (i) Original Mort-
         gagor has, in exercise of its rights under Section 6 of the
         1995 Modification Agreement, conveyed to Mortgagor all of Orig-
         inal Mortgagor's right, title and interest in and to the Orig-
         inal Fee Parcel, (ii) Mortgagee and Original Mortgagor have
         terminated the Corner Parcel Lease, (iii) Mortgagee has con-
         veyed to Mortgagor Mortgagee's fee title in and to the Corner
         Parcel, and (iv) Mortgagor has agreed to assume all of Original
         Mortgagor's obligations arising from and after the date hereof
         under the Notes, the Mortgage and the other Loan Documents pur-
         suant to that certain Assumption, dated as of the date hereof,
         by Mortgagor for the benefit of Mortgagee and Original Mort-
         gagor and their respective successors and assigns (the transac-
         tions described in subparagraphs (i), (ii), (iii) and (iv)
         above shall hereinafter be collectively referred to as the
         "Transfer"); and
          --------

                   WHEREAS, Mortgagor, in consideration of the foregoing
         and of Mortgagee's agreement to consent to the Transfer, has
         agreed to execute and deliver this Agreement.

                   NOW, THEREFORE, in consideration of the premises and
         the mutual covenants herein contained and for other good and
         valuable consideration, the receipt of which is hereby acknowl-
         edged, the parties hereto agree as follows:

                   1.  Representations.  Mortgagor represents and war-
                       ---------------
         rants to Mortgagee that:

                   (a)  except for those matters disclosed in that cer-
              tain Phase I Environment Site Assessment for the Mortgaged
              Premises prepared by Ambient Labs Inc. and dated October
              3, 1997 (the "Environmental Report"), to the best of
                            --------------------
              Mortgagor's knowledge, the Mortgaged Premises do not con-
              tain any hazardous substance, hazardous material, haz-
              ardous waste, toxic substance or solid waste, as defined

                                      - 2 -
<PAGE>
 
              in the Clean Air Act, the Comprehensive Environmental Re-
              sponse, Compensation and Liability Act of 1980, as
              amended, the Resource Conservation and Recovery Act of
              1976, as amended, or the Hazardous Materials Transporta-
              tion Act, any substances or materials listed as hazardous
              or toxic in the United States Department of Transportation
              Table, by the Environmental Protection Agency or any suc-
              cessor agency or under any federal, state or local laws or
              regulations, or any asbestos, asbestos-containing materi-
              als, polychlorinated biphenyls, radon gas, urea formalde-
              hyde foam, pollutants, contaminants, volatile organic com-
              pounds, explosive or radioactive materials (all of the
              above being collectively called "Hazardous Substances")
                                               --------------------
              which, in any such case, violate any federal, state or
              local laws, statutes, rules, codes, regulations or orders
              relating to Hazardous Substances (collectively, "Environ-
                                                               ---------
              mental Laws");
              -----------

                   (b)  except for those matters disclosed in the Envi-
              ronmental Report, to the best of Mortgagor's knowledge,
              there have been no releases or threatened releases at,
              upon, under, or within, nor past or ongoing migration from
              neighboring lands to, the Mortgaged Premises, of any Haz-
              ardous Substances; which, in any such case, violate any
              Environmental Law; and

                   (c)  except as disclosed in the Environmental Cer-
              tificate (hereafter defined), Mortgagor has received no
              notice from any governmental authority or from any tenant
              or other occupant or from any other person with respect to
              any release of any Hazardous Substance at, upon, under, or
              within the Mortgaged Premises.

                   2.  Use of Mortgaged Premises.  Mortgagor will not
                       -------------------------
         use, generate, manufacture, produce, store, release, threaten
         to release, discharge or dispose of (or permit any of the same)
         or permit to exist on or about the Mortgaged Premises or trans-
         port (or permit to be transported) any Hazardous Substance to
         or from the Mortgaged Premises in a manner which would violate
         any Environmental Law, and, if the same exist, Mortgagor will,
         if and to the extent required by any Environmental Law, im-
         mediately remove or cause to be removed from the Mortgaged Pre-
         mises, any such Hazardous Substance.  Mortgagor has not been,
         is not, and will not become involved in, and will not permit,
         operations at, in or about the Mortgaged Premises which could
         lead to the imposition of liability under any Environmental
         Law.  Notwithstanding the foregoing, with respect to any claim
         under this Section 2 arising from the act or omission of any
         tenant, subtenant, licensee or other occupant of the Mortgaged
         Premises, Mortgagor's obligations under this Section 2 shall be


                                      - 3 -
<PAGE>
 
         to promptly and diligently exercise any available remedies
         against such Person in order to cause such Person to cure any
         such default.

                   3.  Compliance.  Mortgagor, at its expense, will
                       ----------
         cause the Mortgaged Premises to comply with all Environmental
         Laws.  Notwithstanding the foregoing, with respect to any claim
         under this Section 3 arising from the act or omission of any
         tenant, subtenant, licensee or other occupant of the Mortgaged
         Premises, Mortgagor's obligations under this Section 3 shall be
         to promptly and diligently exercise any available remedies
         against such Person in order to cause such Person to cure any
         such act or omission.

                   4.  Notice.  Mortgagor will deliver to Mortgagee,
                       ------
         without request or demand, promptly upon becoming aware of any
         presence or release or alleged or threatened presence or re-
         lease of any Hazardous Substance in violation of any Environ-
         mental Law at, upon, under, within or about the Mortgaged Pre-
         mises and immediately upon becoming aware that any regulatory
         agency, governmental body or other person has made any claim or
         taken or threatened to take any action relating to Hazardous
         Substances on or emanating from the Mortgaged Premises or any
         property adjacent to or abutting the Mortgaged Premises, and
         promptly upon receipt of any notice from any governmental
         agency or authority or from any tenant or other occupant or
         from any other person with respect to any such alleged presence
         or release, a notice specifying the facts relating thereto, the
         claim made or action taken or threatened and what action Mort-
         gagor is taking or causing to be taken or proposes to take or
         to cause to be taken with respect thereto.

                   5.  Inspection.  Mortgagor will permit Mortgagee, and
                       ----------
         any representatives designated by Mortgagee, to visit and in-
         spect the Mortgaged Premises or any part thereof, and to sample
         and monitor soil and groundwater and to inspect for Hazardous
         Substances all at such reasonable times and intervals as from
         time to time may be requested and upon reasonable notice and in
         a manner that does not unreasonably interfere with the business
         operations of any tenant or otherwise violates, in any material
         respect, any such tenant's rights under its respective lease.
         In the event that such inspection results from Mortgagee's rea-
         sonable belief that there has been a breach of Mortgagor's rep-
         resentations or covenants set forth in this Agreement, then
         Mortgagor shall bear the reasonable cost of such inspection;
         otherwise, Mortgagee shall bear the cost of such inspection.
         Mortgagee shall not have any duty to make any such inspection
         and shall not incur any liability or obligation for not making
         any such inspection or, once having undertaken any such inspec-
         tion, for not making the same carefully or properly, or for not


                                      - 4 -
<PAGE>
 
         completing the same; nor shall the fact that such inspection
         may not have been made by Mortgagee relieve Mortgagor of any
         obligations that it may otherwise have under the Mortgage or
         this Agreement.

                   6.  Indemnification.  Notwithstanding any provision
                       ---------------
         in the Mortgage, the Notes or any other Loan Document limiting
         or negating the liability of Mortgagor, Mortgagor will protect,
         indemnify, save harmless and defend Mortgagee from and against
         any and all liabilities, obligations, claims, losses, damages,
         penalties, causes of action, reasonable costs and expenses (in-
         cluding, without limitation, reasonable attorneys' fees and
         expenses) (collectively, the "Claims") imposed upon, suffered
                                       ------
         or incurred by or asserted against Mortgagee by reason of the
         presence, release, threatened release or removal of any Hazard-
         ous Substances at, upon, under, within or about the Mortgaged
         Premises or any non-compliance with any legal requirement re-
         lating thereto, whether arising before, during or after en-
         forcement of Mortgagee's rights and remedies upon default and
         whether or not Mortgagor is responsible therefor, including,
         without limitation, any imposition by any governmental author-
         ity of any lien or so-called "super priority lien" upon the
         Mortgaged Premises, clean-up costs, response costs, liability
         for personal injury or property damage or damage to the envi-
         ronment and any fines, penalties and punitive damages with re-
         spect thereto.

                   7.  Environmental Certificates.  In connection with
                       --------------------------
         the 1995 Modification, Original Mortgagor delivered to Mort-
         gagee an Environmental Questionnaire and Certificate (the "En-
                                                                    ---
         vironmental Certificate"), dated October 30, 1995, a copy of
         -----------------------
         which has been delivered to Mortgagor.  Mortgagor covenants
         that

                   (a)  Mortgagor will promptly notify Mortgagee in
              writing if Mortgagor learns that any of the answers on the
              Environmental Certificate either was not true in any mate-
              rial respect when made or is no longer true in any mate-
              rial respect; and

                   (b)  if any event shall occur which causes any of the
              answers on the Environmental Certificate to be no longer
              true or if Mortgagee shall learn that any of the answers
              on the Environmental Certificate were not true when made,
              such event or occurrence shall, if not cured within twenty
              (20) days after notice of such event or occurrence is de-
              livered to Mortgagor (whether from Mortgagee or any other
              party), constitute an Event of Default under the Mortgage,
              giving the Mortgagee all the rights and remedies available
              to it upon the occurrence of an Event of Default, provided


                                      - 5 -
<PAGE>
 
              that if such cure cannot with due diligence and dispatch
              be effected within such 20 day period, then such 20 day
              period shall be deemed extended for such period as Mort-
              gagor shall be attempting to effect such cure with due
              diligence and dispatch.

                   8.  Survival.  The obligations of Mortgagor under
                       --------
         Section 6 of this Agreement shall survive any payment of the
         Notes, any discharge, satisfaction, release or assignment of
         the Mortgage, any transfer of the Mortgaged Premises or any
         part thereof, any exercise of remedies by Mortgagee, including,
         without limitation, the appointment of a receiver, any foreclo-
         sure of the Mortgage or any transfer of the Mortgaged Premises
         (or any part thereof) by deed in lieu of foreclosure and any
         other event or circumstance whatsoever.

                   9.   Personal Liability.  In accordance with Sec-
                        ------------------
         tion 6 of the 1997 Modification Agreement, Mortgagor shall be
         personally liable for all of its representations, covenants and
         obligations set forth in this Agreement.

                   10.  Binding Effect.  This Agreement shall inure to
                        --------------
         the benefit of Mortgagee and its successors and assigns, and
         shall be binding upon Mortgagor and its respective successors
         and assigns.

                   11.  Counterparts.  This Agreement may be executed in
                        ------------
         several counterparts, each of which shall constitute one and
         the same instrument.

                   12.  Notices.  All notices, demands, requests, con-
                        -------
         sents, approvals and other instruments under this Agreement
         shall be in writing and shall be deemed to have been actually
         or properly given if and when mailed by first-class registered
         or certified mail, return receipt requested, postage prepaid,
         or by personal delivery (with a receipt obtained therefor) or
         recognized overnight courier service, addressed (a) if to Mort-
         gagor, to Boston Properties Limited Partnership, 599 Lexington
         Avenue, New York, New York 10022, Attention, Robert Selsam, or
         such other address as Mortgagor may have designated by notice
         to Mortgagee, with a copy to Goodwin, Proctor & Hoar, LLP, Ex-
         change Place, Boston, Massachusetts 02109, Attention, Edward M.
         Schulman, Esq., with a copy to Weil, Gotshal & Manages LLP, 767
         Fifth Avenue, New York, New York 10153, Attention, Alan J. 
         Pomerantz, Esq. or to such other firms as Mortgagor may direct
         in writing, and (b) if to Mortgagee, to John Hancock Mutual
         Life Insurance Company, John Hancock Place, P.O. Box 111, Bos-
         ton, Massachusetts 02117, Attention, Real Estate Investment
         Group, Floor T-52, or at such other address as Mortgagee may
         have designated by notice to Mortgagor.


                                      - 6 -
<PAGE>
 
                   13.  Miscellaneous.  Mortgagor agrees that:
                        -------------

                   (a)  If any term of this Agreement or any application
              thereof shall be declared invalid, illegal or unenforce-
              able, the remainder of this Agreement and any other ap-
              plication of such terms shall not be affected thereby.

                   (b)  No failure or delay on the part of Mortgagee to
              exercise any power, right or privilege under this Agree-
              ment shall impair any such power, right or privilege or be
              construed to be a waiver of any default or any acquies-
              cence therein, nor shall any single or partial exercise of
              such power, right or privilege preclude other or further
              exercise thereof or of any right, power or privilege.

                   (c)  No provision of this Agreement may be changed,
              waived, discharged or terminated except by an instrument
              in writing signed by the party against whom enforcement of
              the change, waiver, discharge or termination is sought.

                   (d)  This Agreement shall be governed and construed
              in accordance with the laws of the State of New York.

                   (e)  Mortgagor shall pay all reasonable attorneys'
              fees and other reasonable costs and expenses which may be
              incurred by Mortgagee in the enforcement of this Agree-
              ment.







                   [Remainder of Page Intentionally Left Blank]




                                      - 7 -
<PAGE>
 
                   IN WITNESS WHEREOF, the undersigned has caused this
         Environmental Indemnity and Agreement to be duly executed and
         delivered by its duly authorized officers as of the date first
         above written.


                                      MORTGAGOR:

                                      BOSTON PROPERTIES LIMITED
                                      PARTNERSHIP,
                                        A Delaware Limited Partnership

                                      By:  Boston Properties, Inc.,
                                           General Partner

                                           By:  /s/ Robert Selsam
                                                ----------------------------
                                                Name:  Robert Selsam
                                                Title: Senior Vice President
<PAGE>
 
         STATE OF NEW YORK   )
                             :    ss:
         COUNTY OF NEW YORK  )



                   On this ____ day of November, 1997, before me person-
         ally came ___________________, to me known to be the person(s)
         who executed the foregoing instrument, and who, being duly
         sworn by me, did depose and say that he/she is the
         _________________ of Boston Properties, Inc., a Delaware corpo-
         ration, the general partner of Boston Properties Limited Part-
         nership, a limited partnership duly organized under the laws of
         the State of Delaware, having an address at 599 Lexington Ave-
         nue, New York, New York 10022; and that he/she has authority to
         sign the foregoing instrument for and on behalf of Boston Prop-
         erties Limited Partnership, and he/she acknowledged to me that
         he/she executed the same as the act and deed of said partner-
         ship for the use and purposes therein mentioned.



                                        -----------------------------
                                                Notary Public
