<PAGE>
 
                                                                   EXHIBIT 10.15


                                     LEASE

                                    BETWEEN

           RIVERFRONT OFFICE PARK ASSOCIATES II LIMITED PARTNERSHIP

                                   Landlord



                                      AND



                       FOCAL COMMUNICATIONS CORPORATION

                                    Tenant

                               ONE MAIN STREET,
                           CAMBRIDGE, MASSACHUSETTS
<PAGE>
 
                                     INDEX
                                     -----

<TABLE> 
<S>                                                                                              <C> 
1.       REFERENCE DATA.........................................................................  1 
                                                                                                   
                                                                                                   
2.       DESCRIPTION OF DEMISED PREMISES........................................................  3 
         2.1 Demised Premises...................................................................  3 
         2.2 Appurtenant Rights.................................................................  3 
         2.3 Reservations.......................................................................  3 
         2.4 Roof Antenna; Telecommunications Equipment License.................................  3 
                                                                                                   
                                                                                                   
3.       TERM OF LEASE; OPTION TO EXTEND TERM...................................................  5 
         3.1 Habendum...........................................................................  5 
         3.2 Term Commencement Date.............................................................  5 
         3.3 Option to Extend...................................................................  5 
         3.4 Right of First Offer...............................................................  6 
                                                                                                   
                                                                                                   
4.       CONDITION OF PREMISES; TENANT'S FIT-UP ALLOWANCE; TENANT'S ACCESS......................  6 
         4.1 Condition of Premises..............................................................  6 
         4.2 Tenant Allowance for Fit-Up Work...................................................  6 
         4.3 Tenant's Access....................................................................  7 
                                                                                                   
                                                                                                   
5.       USE OF PREMISES........................................................................  7 
         5.1 Permitted Use......................................................................  7 
         5.2 Prohibited Uses....................................................................  7 
         5.3 Licenses and Permits...............................................................  7 
                                                                                                   
                                                                                                   
6.       RENT...................................................................................  8 
         6.1 Yearly Fixed Rent..................................................................  8 
         6.2 Taxes..............................................................................  8 
         6.3 Operating Expenses.................................................................  8 
         6.4 Tenant's Proportionate Share.......................................................  9 
         6.5 Payment to Mortgagee...............................................................  9 
         6.6 Right to Audit Records.............................................................  9 
                                                                                                
                                                                                                
7.       UTILITIES AND LANDLORD'S SERVICES...................................................... 10
         7.1 Electricity........................................................................ 10
         7.2 Water Charges...................................................................... 11
</TABLE> 
<PAGE>
 
<TABLE> 
<S>                                                                                           <C> 
         7.3 Heat and Air Conditioning.......................................................... 11
         7.4 Additional Heat, Cleaning and Air Conditioning Services............................ 11
         7.5 Repairs and Other Services......................................................... 12
         7.6 Interruption or Curtailment of Services............................................ 12


8.       CHANGES OR ALTERATIONS BY LANDLORD..................................................... 12


9.       FIXTURES, EQUIPMENT AND IMPROVEMENTS - REMOVAL BY TENANT............................... 13
                                                                                                
                                                                                                
10.      ALTERATIONS AND IMPROVEMENTS BY TENANT; RESTORATION TO SHELL CONDITION................. 13


11.      TENANT'S CONTRACTORS - MECHANICS' AND OTHER LIENS - 
         STANDARD OF TENANT'S PERFORMANCE - COMPLIANCE WITH LAWS................................ 14 


12.      REPAIRS AND SECURITY BY TENANT......................................................... 14
                                                                                                   

13.      INSURANCE, INDEMNIFICATION, EXONERATION AND EXCULPATION................................ 15
         13.1 Insurance......................................................................... 15
         13.2 Certificates of Insurance......................................................... 15
         13.3 General........................................................................... 15
         13.4 Property of Tenant................................................................ 16
         13.5 Bursting of Pipes, etc............................................................ 16
         13.6 Repairs and Alterations - No Diminution of Rental Value........................... 17
                                                                                                   
                                                                                                   
14.      ASSIGNMENT, MORTGAGING, SUBLETTING, ETC................................................ 17
         14.1 Restrictions...................................................................... 17
         14.2 Requests to Assign or Sublet...................................................... 17
         14.3 Exceptions........................................................................ 18
         14.4 Excess Rent....................................................................... 18
         14.5 Recapture......................................................................... 18
         14.6 Further Documentation............................................................. 20
         14.7 General........................................................................... 20
                                                                                                   
                                                                                                   
15.      MISCELLANEOUS COVENANTS................................................................ 21
         15.1 Rules and Regulations............................................................. 21
         15.2 Access to Premises - Shoring...................................................... 22 
</TABLE> 
<PAGE>
 
<TABLE> 
<S>                                                                                              <C> 
         15.3 Accidents to Sanitary and Other Systems........................................... 22
         15.4 Signs, Blinds and Drapes.......................................................... 23
         15.5 Estoppel Certificate.............................................................. 23
         15.6 Prohibited Items.................................................................. 23
         15.7 Requirements of Law; Fines and Penalties.......................................... 23
         15.8 Tenant's Acts - Effect on Insurance............................................... 24
         15.9 Hazardous Materials............................................................... 24
         15.10 Miscellaneous.................................................................... 25
                                                                                                  
                                                                                                  
16.      DAMAGE BY FIRE, ETC.................................................................... 25
                                                                                                  
                                                                                                  
17.      WAIVER OF SUBROGATION.................................................................. 26
                                                                                                  
                                                                                                  
18.      CONDEMNATION - EMINENT DOMAIN.......................................................... 27
                                                                                                  
                                                                                                  
19.      DEFAULT................................................................................ 28
         19.1 Conditions of Limitation - Re-entry - Termination................................. 28
         19.2 Damages - Assignment for Benefit of Creditors..................................... 29
         19.3 Damages - Termination............................................................. 29
         19.4 Fees and Expenses................................................................. 30
         19.5 Landlord's Remedies Not Exclusive................................................. 31
         19.6 Grace Period...................................................................... 31
                                                                                                  
                                                                                                  
20.      END OF TERM - ABANDONED PROPERTY....................................................... 31
                                                                                                  
                                                                                                  
21.      RIGHTS OF MORTGAGEES................................................................... 32
         21.1 Superiority of Lease.............................................................. 32
         21.2 Entry and Possession.............................................................. 32
         21.3 Right to Cure..................................................................... 32
         21.4 Prepaid Rent...................................................................... 33
         21.5 Continuing Offer.................................................................. 33
         21.6 Subordination..................................................................... 33
         21.7 Limitations on Liability.......................................................... 33
                                                                                                  
                                                                                                  
22.      QUIET ENJOYMENT........................................................................ 34
                                                                                                  
                                                                                                  
23.      ENTIRE AGREEMENT - WAIVER - SURRENDER.................................................. 34
</TABLE> 
<PAGE>
 
<TABLE> 
<S>                                                                                              <C>  
         23.1 Entire Agreement.................................................................. 34
         23.2 Waiver by Landlord................................................................ 34
         23.3 Surrender......................................................................... 34
                                                                                                   
                                                                                                   
24.      INABILITY TO PERFORM - EXCULPATORY CLAUSE.............................................. 35
                                                                                                   
                                                                                                   
25.      BILLS AND NOTICES...................................................................... 35
                                                                                                   
                                                                                                   
26.      PARTIES BOUND - SEIZING OF TITLE....................................................... 36
                                                                                                   
                                                                                                   
27.      MISCELLANEOUS.......................................................................... 36
         27.1 Separability...................................................................... 36
         27.2 Captions.......................................................................... 37
         27.3 Broker............................................................................ 37
         27.4 Governing Law..................................................................... 37
         27.5 Assignment of Rents............................................................... 37
         27.6 Parking........................................................................... 37
         27.7 Notice of Lease................................................................... 38
         27.8 Financial Statements.............................................................. 38
         27.9 Holding Over...................................................................... 38 
</TABLE> 

EXHIBIT A                  Description of Demised Premises
EXHIBIT B                  Description of Land
EXHIBIT C                  Audit Procedures
EXHIBIT D                  Procedure for Allocation of Costs of
                            Electric Power Usage by Tenants
EXHIBIT E                  Cleaning Specifications
EXHIBIT F                  Rules and Regulations
<PAGE>
 
                              AGREEMENT OF LEASE

          AGREEMENT OF LEASE made as of the 6th day of January, 1999, by and
between RIVERFRONT OFFICE PARK ASSOCIATES II LIMITED PARTNERSHIP, a
Massachusetts limited partnership (hereinafter referred to as "Landlord") and
FOCAL COMMUNICATIONS CORPORATION, a Delaware corporation (hereinafter referred
to as "Tenant").

                             W I T N E S S E T H:

          Landlord hereby leases to Tenant and Tenant hereby hires from Landlord
a portion of the eleventh (11th) floor, as shown on the plan attached hereto as
Exhibit A and made a part hereof (hereinafter referred to as the "Premises" or
---------                                                                     
the "Demised Premises") contained in the building known and numbered as One Main
Street, Cambridge, Massachusetts (hereinafter referred to as the "Building").

1.   REFERENCE DATA

     Each reference in this Lease to any of the terms and titles contained in
this Article shall be deemed and construed to incorporate the data stated
following that term or title in this Article.

(1)  Additional Rent:      Sums or other charges payable by Tenant to Landlord
                           under this Lease, other than Yearly Fixed Rent.

(2)  Business Day:         All days except Saturdays, Sundays, and days defined
                           as "Legal Holidays" for the entire state under the
                           laws of the Commonwealth of Massachusetts.

(3)  Consumer Price Index: The United States Department of Labor's Bureau of
                           Labor Statistics, Consumer Price Index (1982-
                           1984=100), All Urban Consumers, All Items, Boston,
                           Massachusetts, or the successor to such index.

(4)  Land:                 The parcel of land described on Exhibit B attached
                                                           ---------         
                           hereto and made a part hereof.

(5)  Landlord's Address:   c/o Codman Management Company, Inc., One Main
                           Street, Cambridge, Massachusetts 02142.

(6)  Mortgage:             A mortgage, deed of trust, trust indenture, or other
                           security instrument of record creating an interest in
                           or affecting title to the Land or Building or any
                           part thereof, including the leasehold mortgage, and
                           any and all renewals, modifications, consolidations
                           or extensions of any such instrument.
                           
<PAGE>
 
(7)  Mortgagee:           The holder of any Mortgage.
 
(8)  Parking Spaces:      Thirteen (13).
 
(9)  Property:            The Land and Building.
 
(10) Rent:                Yearly Fixed Rent and Additional Rent.
 
(11) Rentable Area of the
     Demised Premises:    13,274 square feet.
 
(12) Tenant's Address:    One Main Street, Cambridge, Massachusetts 02142.
 
(13) Term Commencement
     Date:                As defined in Section 3.2.
 
(14) Term of This Lease:  As defined in Section 3.1.
 
(15) Termination Date:    As defined in Section 3.1.

(16) Use of Demised
     Premises:            General office purposes, computer and
                          telecommunications operations, and all other uses
                          incidental and related thereto consistent with that in
                          first-class office buildings in the greater Boston
                          area (the "Permitted Use").

(17) Yearly Fixed Rent:   (a)  During the first five (5) years of the Term of
                               ----------------------------------------------
                               this Lease:
                               ---------- 

                               $530,960.00 per annum, based upon a rental rate
                               of $40.00 per rentable square foot per annum.

                          (b)  During the sixth (6th) year through and including
                               -------------------------------------------------
                               the tenth (10th) year of the Term of this Lease:
                               ----------------------------------------------- 

                               An amount per annum equal to the product of (i)
                               $530,960.00 multiplied by (ii) a fraction, the
                               denominator of which shall be the most recent
                               Consumer Price Index figure published prior to
                               the Term Commencement Date, and the numerator of
                               which shall be the Consumer Price Index for the
                               month of December, 2003 (or if not published for
                               December, 2003, the then most recent previous

                                       2
<PAGE>
 
                              month for which the Index is published).


2.   DESCRIPTION OF DEMISED PREMISES

     2.1 Demised Premises.  The Demised Premises are that portion of the
         ----------------                                                  
Building as described above (as the same may from time to time be constituted
after changes therein, additions thereto and eliminations therefrom pursuant
hereto).

     2.2 Appurtenant Rights.  Tenant shall have, as appurtenant to the
         ------------------                                              
Demised Premises, (a) rights to use in common, subject to reasonable rules from
time to time made by Landlord of which Tenant is given notice, those common
roadways, walkways, elevators, hallways and stairways necessary for access to
that portion of the Building occupied by the Demised Premises, (b) the sole and
exclusive right to use and maintain the dedicated electric riser, panel board,
and related switching equipment servicing the 11th floor switch room, and (c)
the sole and exclusive right to use and maintain the chiller water plant serving
the Demised Premises located in part on the 14th floor and in part on the roof
of the Building.  Tenant shall also be provided by Landlord, such space on the
roof of the Building, or elsewhere if acceptable to Landlord and Tenant, as
shall be reasonably necessary for the placement of mechanical equipment
necessary to serve the heating, ventilating and emergency back-up air
conditioning system of the Tenant for the Demised Premises and for Tenant's
computer operations, and such additional rights of access thereto from the
Demised Premises for the purpose of installing, repairing and maintaining same;
provided, however, that if during the Term of this Lease, Tenant's mechanical
and equipment requirements to serve the heating, ventilating and air
conditioning system of the Demised Premises and for Tenant's computer operations
change such that the space then provided by Landlord to Tenant for such purposes
is no longer required by Tenant, then, in such event, Tenant shall so notify
Landlord in writing, and such space shall revert back to Landlord with Tenant
relinquishing any claim or right of access thereto.

     2.3 Reservations.  All the perimeter walls of the Demised Premises except
         ------------                                                           
the inner surfaces thereof, any balconies, terraces or roofs adjacent to the
Demised Premises, and any space in or adjacent to the Demised Premises used for
shafts, stacks, pipes, conduits, wires and appurtenant fixtures, fan rooms,
ducts, electric or other utilities, sinks or other building facilities, and the
use thereof, as well as the right of access through the Demised Premises for the
purpose of operation, maintenance, decoration and repair as hereinafter
provided, are expressly reserved to Landlord.

     2.4 Roof Antenna; Telecommunications Equipment License.  (a) Landlord
         --------------------------------------------------                 
grants to Tenant a license during the Term of this Lease, and only during the
Term of this Lease, subject to the terms and conditions of this Lease, at
Tenant's cost and expense, to install, operate and maintain on the roof of the
Building, at a location and within an area (the "Licensed Roof Area") designated
by Landlord, a small GPS antenna (the "Licensed Antenna").  Tenant, at its cost
and expense, shall be responsible for the preparation and submission to Landlord
of architectural, electrical and mechanical construction drawings, plans and
specifications (collectively, the "Plans") necessary for the configuration and
installation of the Licensed Antenna.  The Plans shall be subject to

                                       3
<PAGE>
 
Landlord's prior written approval, which approval shall not be unreasonably
withheld or delayed. Tenant shall be responsible at the cost and expense for
obtaining all licenses, permits and consents necessary to undertake and complete
the construction of the work contemplated by the Plans. Tenant shall be
responsible at its cost and expense for the construction of all improvements
contemplated by the Plans (the "Improvements"), which shall include appropriate
lightning protection on the Licensed Roof Antenna, provided that all contractors
and construction shall be subject to Landlord's prior written approval, which
approval shall not be unreasonably withheld or delayed, and, provided further
that Landlord shall have the right (but not the obligation) to supervise all
construction, installation, maintenance or repair work undertaken by Tenant
under such license. In no event shall Tenant's construction of the Improvements
pursuant to the Plans result in any penetration of the roof or roof membrane of
the Building (i.e. Tenant shall utilize the existing weather head) unless
otherwise expressly agreed to in writing by Landlord and then only by a
contractor designated by Landlord. On or before the Termination Date, Tenant, at
its sole and expense, shall remove the Improvements and repair any damage to the
Building (including, without limitation, the roof) occasioned by said
installation and/or removal. The Licensed Roof Area shall be used by Licensee
solely for the operation, maintenance and installation of Improvements and for
no other purposes. If any governmental license or permit shall be required for
the proper and lawful conduct of Tenant's business, and if the failure to secure
such license or permit would in any way adversely affect Landlord, Tenant, at
Tenant's cost and expense, shall duly procure and thereafter maintain and remain
in compliance with the terms and conditions such license or permit.

          (b) Subject to the terms and conditions of this Lease, Landlord grants
to Tenant a license during the Term of this Lease, to install, operate and
maintain such telecommunications equipment, cables, junction boxes and not more
than four (4) 4" conduits (collectively, "Telecommunications Equipment and
Cabling") within dedicated portions of the Building's riser systems and utility
areas of the Building as shall be necessary to provide telecommunications
services and point-to-point transmission to tenants and occupants of the
Building.  As of the Commencement Date there are currently two (2) 4" conduits
that may be used by Tenant and Landlord agrees that Tenant shall have the right
to install an additional two (2) additional 4" conduits (the "Additional
Conduits") for a total of four (4) 4" conduits that may be used by Tenant.  The
installation, operation and maintenance of such Telecommunications Equipment and
Cabling shall be subject in each instance to Landlord's prior written approval,
which approval shall not be unreasonably withheld or delayed.  Tenant shall at
all times install, maintain and operate the Telecommunications Equipment and
Cabling in accordance with recognized industry standards and all requirements of
law, regulation or ordinance now or hereafter applicable.  Without limiting the
generality of the foregoing, Tenant shall provide all necessary safeguards
against radiation and other potential health hazards.  The Telecommunications
Equipment and Cabling shall at all times be operated in such a manner as to not
cause interference of any kind with present or future transmissions or reception
of radio, television, telephone or other telecommunications equipment in the
Building.  Tenant shall have the right, at its sole risk and expense, to permit
its customers to locate telecommunications equipment in the Demised Premises,
subject to the provisions of Section 5.2 ("Colocation").  At Landlord's written
election, Tenant, at Tenant's sole cost and expense, prior to the end of the
Term of this Lease, shall remove all

                                       4
<PAGE>
 
Telecommunications Equipment and Cabling located within the Building's riser
systems and utility areas of the Building, provided, however, that
notwithstanding Landlord's election that any such Telecommunications Equipment
and Cabling not be removed (if Landlord shall so elect), Tenant shall have the
right to remove all Telecommunications Equipment and Cabling.

          (c) In no event shall any material or equipment be installed which
shall cause the Demised Premises to be subject to any lien, charge, mortgage or
other encumbrance of any kind whatsoever. Any mechanic's lien filed against the
Building for work claimed to have been done for, or materials claimed to have
been furnished to Tenant, shall be discharged by Tenant within thirty (30) days
thereafter, at the expense of Tenant, by filing with bond required by law or
otherwise. If Tenant fails so to discharge any lien, Landlord may do so at
Tenant's expense and Tenant shall reimburse Landlord for any reasonable expense
or cost incurred by Landlord in so doing within fifteen (15) days after
rendition of a bill therefor. All installations or work done by Tenant under the
licenses granted pursuant to this Section 2.4 shall be at Tenant's own cost and
expense and shall at all times comply with (i) all applicable laws, rules,
orders and regulations of governmental authorities having jurisdiction thereof;
(ii) all applicable orders, rules and regulations of any Board of Fire
Underwriters, or any other body hereafter constituted exercising similar
functions, and governing insurance rating bureaus; and (iii) the Plans. Tenant
shall promptly repair all damage to the Building or any part thereof or any
personal property therein arising from any activities conducted by or on behalf
of Tenant pursuant to the licenses granted pursuant to this Section 2.4.

3.   TERM OF LEASE; OPTION TO EXTEND TERM; RIGHT OF FIRST OFFER

     3.1 Habendum.  TO HAVE AND TO HOLD the Demised Premises for a term of
         --------                                                            
ten (10) years commencing on January 6, 1999 and ending on the day immediately
prior to the tenth (10th) anniversary thereof (the "Term of this Lease"), or on
such earlier date upon which said Term may expire or be terminated pursuant to
any of the conditions of limitation or other provisions of this Lease or
pursuant to law (which date for the termination of the term hereof shall
hereafter be called the "Termination Date").

     3.2 Term Commencement Date.  The Term Commencement Date shall be
         ----------------------                                          
January 6, 1999.

     3.3 Option to Extend.  Tenant may extend the Term of this Lease for up to
         ----------------                                                       
two (2) additional five (5) year period(s) by giving notice to Landlord of its
election to extend at least fourteen (14) months prior to the end of the
original Term or the end of the first five year extension period, as the case
may be.  The Yearly Fixed Rent payable by Tenant with respect to each such
extension period shall be equal to the greater of (a) the then fair market
                                   --------------                         
rental value of the Demised Premises for use for the Permitted Use, or (b) (i)
                                                                    --        
the Yearly Fixed Rent for calendar 2008, in the case of determination of the
Yearly Fixed Rent payable with respect to the first five (5) year extension
period and (ii) the Yearly Fixed Rent for calendar 2013, in the case of
determination of the Yearly Fixed Rent payable with respect to the second five
(5) year extension period.  As used herein, "fair market rental value" shall
mean (1) as the same may be mutually

                                       5
<PAGE>
 
agreed by Landlord and Tenant; provided, however that (2) if they have not so
agreed in writing within two (2) months following the exercise of the option
then said fair market value shall be determined by appraisers, one to be chosen
by Landlord, one to be chosen by Tenant, and a third to be selected by the two
first chosen. The unanimous written decision of the two first chosen, without
selection and participation of a third appraiser, or otherwise the written
decision of a majority of three appraisers chosen and selected as aforesaid,
shall be conclusive and binding upon Landlord and Tenant. Landlord and Tenant
shall each notify the other of its chosen appraiser within thirty (30) days
following expiration of the aforesaid two (2) month period and, unless such two
appraisers shall have reached a unanimous decision within seventy-five (75) days
from said expiration, they shall within a further fifteen (15) days elect a
third appraiser and notify Landlord and Tenant thereof. Landlord and Tenant
shall each bear the expense of the appraiser chosen by it and shall equally bear
the expense of the third appraiser (if any).

     3.4 Right of First Offer.  In the event that at any time during the Term
         --------------------                                                  
of this Lease any rentable space on the eleventh (11th) floor of the Building
then occupied by COM/Energy Services Company becomes or is to become available
for rental, and in the further event Tenant is not then in default of its
material obligations to Landlord under this Lease, Landlord shall make a written
offer to lease the same to Tenant, stating the Rent that Landlord will accept
(which Rent shall reflect Landlord's judgment as to then fair market rental
value of such space) and all other material terms and conditions of the proposed
lease, and Tenant shall have a right of first refusal to lease such space by
giving notice to Landlord to such effect within fifteen (15) Business Days after
notice of such offer.  If such notice of acceptance by Tenant is not so given,
then Landlord shall be free thereafter to lease such space to a third party on
such terms and conditions as Landlord shall determine.  In any case in which
Tenant shall have waived said right of first refusal, or Tenant shall have
failed to timely exercise such right, then Tenant shall, on request of Landlord,
execute and deliver in recordable form an instrument indicating such waiver or
expiration, which instrument shall be conclusive in favor of all persons relying
thereon in good faith.

4.   CONDITION OF PREMISES; TENANT'S FIT-UP ALLOWANCE; TENANT'S ACCESS

     4.1 Condition of Premises.  The Demised Premises are leased in their
         ---------------------                                             
present "AS IS" condition.  Without limiting the foregoing in any respect,
Tenant hereby acknowledges and agrees that Landlord shall have no responsibility
for, or liability with respect to, the condition and/or operation (or failure to
operate or malfunction) of any electronic and other equipment which may be
located on the Demised Premises on the Term Commencement Date.

     4.2 Tenant Allowance for Fit-Up Work.  Landlord shall provide to Tenant a
         --------------------------------                                       
tenant fit-up cash reimbursement allowance of up to $132,274 ($10 per square
foot) to reimburse Tenant for out-of-pocket costs incurred by Tenant for tenant
fit-up work and tenant improvements to the Demised Premises and associated
architectural and engineering fees, payable to Tenant by Landlord within thirty
(30) days of receipt by Landlord of paid invoices evidencing amounts paid by
Tenant and invoiced by Tenant to Landlord for tenant fit-up work completed prior
to June 1,

                                       6
<PAGE>
 
1999. Tenant shall have the right, subject to Landlord's approval, to install,
at Tenant's cost and expense, window treatments or interior walls to seal and
stabilize the environmental conditions within the technical space in the Demised
Premises. All such tenant fit-up work shall be subject to and accomplished in
compliance with Article 10 of this Lease.

     4.3 Tenant's Access.  Tenant shall be permitted access to the Demised
         ---------------                                                    
Premises immediately following execution of this Lease to undertake tenant fit-
up work and to prepare the Demised Premises for Tenant's occupancy.  Such access
shall be deemed to be pursuant to a license from Landlord to Tenant and shall be
at the risk of Tenant.  In no event shall Tenant interfere with any construction
work being performed by or on behalf of Landlord in or around the Building;
without limiting the generality of the foregoing, Tenant shall comply with all
instructions issued by Landlord's agents relative to the moving of Tenant's
equipment and other property into the Demised Premises and shall pay any
reasonable fees or costs imposed in connection therewith.

5.   USE OF PREMISES

     5.1 Permitted Use.  Except as otherwise agreed by Landlord, Tenant,
         -------------                                                     
during the Term of this Lease, shall occupy and use the Demised Premises for the
Permitted Use set forth in Article 1 and for no other purpose.  Notwithstanding
the foregoing, as long as Tenant continues to operate its business in the
Demised Premises for the Permitted Use, even though no personnel occupy the
Demised Premises, Tenant shall be deemed to have met its obligation hereunder to
occupy and use the Demised Premises.  Service and utility areas (whether or not
a part of the Premises) shall be used only for the particular purpose for which
they are designated.

     5.2 Prohibited Uses.  Tenant shall not use, or suffer or permit the use
         ---------------                                                       
of, or suffer or permit anything to be done in or anything to be brought into or
kept in, the Demised Premises or any part thereof (i) which would violate any of
the covenants, agreements, terms, provisions and conditions of this Lease, (ii)
for any unlawful purposes or in any unlawful manner, or (iii) which, in the
reasonable judgment of Landlord shall in any way (a) impair or tend to impair
the appearance or reputation of the Building, (b) impair or interfere with or
tend to impair or interfere with any of the Building services or the proper and
economic heating, cleaning, air conditioning or other servicing of the Building
or Demised Premises, or with the use of any of the other areas of the Building,
or (c) occasion discomfort, inconvenience or annoyance to any of the other
tenants or occupants of the Building, whether through the transmission of noise
or odors or otherwise.  Without limiting the generality of the foregoing, no
food shall be prepared or served for consumption on or about the Demised
Premises; no intoxicating liquors or alcoholic beverages shall be sold on or
about the Demised Premises; no lottery tickets (even where the sale of such
tickets is not illegal) shall be sold and no gambling, betting or wagering shall
otherwise be permitted on or about the Demised Premises; no loitering shall be
permitted in or about the Demised Premises; and no loading or unloading of
supplies or other material to or from the Demised Premises shall be permitted on
the Land except at times and in locations to be designated by Landlord.  The
Demised Premises shall be maintained in a sanitary condition, and all kept free
of rodents and vermin.  All trash and rubbish shall be suitably stored in the
Demised Premises or

                                       7
<PAGE>
 
other locations designated by Landlord from time to time.

     5.3 Licenses and Permits.  If any governmental license or permit shall
         --------------------                                                  
be required for the proper and lawful conduct of Tenant's business, and if the
failure to secure such license or permit would in any way adversely affect
Landlord, Tenant, at Tenant's expense, shall duly procure and thereafter
maintain such license or permit and submit the same to inspection by Landlord.
Tenant,  at Tenant's expense, shall at all times comply with the terms and
conditions of each such license or permit.

6.   RENT

     6.1 Yearly Fixed Rent.  Tenant shall pay to Landlord, without any set-
         -----------------                                                    
off or deduction, at Landlord's office, or to such other person or at such other
place as Landlord may designate by notice to Tenant, the Yearly Fixed Rent set
forth in Article 1.  All Yearly Fixed Rent shall be paid in equal monthly
installments in advance on or before the first business day of each calendar
month during the Term of this Lease and shall be apportioned for any fraction of
a month in which the Term Commencement Date or the last day of the Term of this
Lease may fall.

     6.2 Taxes.  Tenant shall pay to Landlord as Additional Rent a
         -----                                                     
proportionate share (as defined in Section 6.4) of all real estate taxes
(including without limitation all betterment assessments and charges in lieu of
such taxes and any tax on any fixture (other than a Tenant fixture) installed in
the Building, even if taxed as personal property) imposed against the Building
and the Land in any fiscal tax year, in excess of the amount of said real estate
taxes imposed against the Building and the Land for the fiscal tax year ending
June 30, 1999, prorated with respect to any portion of a fiscal year in which
the term of this Lease begins or ends.  Such payments shall be due and payable
within thirty (30) days after Tenant shall have received a copy of the relevant
tax bills.  If Landlord shall receive any refund of real estate taxes of which
Tenant has paid a portion pursuant to this Section, then, out of any balance
remaining after deducting Landlord's expenses incurred in obtaining such refund,
Landlord shall pay to Tenant the same proportionate share of said balance,
prorated as set forth above.  Tenant shall, if, as and when demanded by Landlord
and with each monthly installment of Fixed Rent, make tax fund payments to
Landlord.  "Tax Fund Payments" refer to such payments as Landlord shall
reasonably determine to be sufficient to provide in the aggregate a fund
adequate to pay, when they become due and payable, all payments required from
Tenant under this Section.  In the event that said tax fund payments are not
adequate to pay Tenant's share of such taxes, Tenant shall pay to Landlord the
amount by which such aggregate is less than the amount of said share, such
payment to be due and payable at the time set forth above.  Any surplus tax fund
payments shall be accounted for to Tenant after payment by Landlord of the taxes
on account of which they were made, and shall be promptly refunded to Tenant.

     6.3 Operating Expenses.  Tenant shall pay to Landlord as Additional Rent
         ------------------                                                     
a proportionate share (as defined in Section 6.4) of all costs and expenses
incurred by Landlord in the operation and maintenance of the Building and the
Land in accordance with generally accepted operational and maintenance
procedures in any calendar year, in excess of the amount of said costs and

                                       8
<PAGE>
 
expenses incurred by the Landlord in the operation and maintenance of the
Building and the Land during the calendar year ending December 31, 1998,
including, without limiting the generality of the foregoing, all such costs and
expenses in connection with (1) insurance, license fees, janitorial service,
landscaping, and snow removal, (2) wages, salaries, management fees, employee
benefits, payroll taxes, on-site office expenses, administrative and auditing
expenses, and equipment and materials for the operation, management, and
maintenance of said Property, (3) any capital expenditure (amortized, with
interest, on such reasonable basis as Landlord shall determine) made by Landlord
for the purpose of reducing other operating expenses or complying with any
governmental requirement, (4) the furnishing of heat, air conditioning,
utilities, and any other service to the common areas of the Building (i.e.,
areas not constituting a part of the demised premises of any tenant in the
Building), (5) the operation and servicing of any computer system installed to
regulate Building equipment, and (6) the furnishing of the repairs and services
referred to in Section 7.3 (the foregoing being hereinafter referred to as
"operating expenses").  As soon as Tenant's share of operating expenses with
respect to any calendar year can be determined, the same will be certified by
Landlord to Tenant and will become payable to Landlord within thirty (30) days
following such certification, subject to proration with respect to any portion
of a calendar year in which the Term of this Lease begins or ends.  Tenant
shall, if, as and when demanded by Landlord and with each monthly installment of
Yearly Fixed Rent, make operating fund payments to Landlord.  "Operating Fund
Payments" refer to such payments as Landlord shall reasonably determine to be
sufficient to provide in the aggregate a fund adequate to pay, when they become
due and payable, all payments required from Tenant under this Section.  In the
event that operating fund payments are so demanded, and if the aggregate of said
operating fund payments is not adequate to pay Tenant's share of operating
expenses, Tenant shall pay to Landlord the amount by which such aggregate is
less than the amount of said share, such payment to be due and payable at the
time set forth above.  Any surplus operating fund payments shall be accounted
for to Tenant after such surplus has been determined, and shall be refunded to
Tenant promptly.

     6.4 Tenant's Proportionate Share.  Tenant's proportionate share of
         ----------------------------                                      
taxes pursuant to Section 6.2 and operating expenses pursuant to Section 6.3,
respectively, shall be 4.07% (13,274 square feet divided by 326,470 square feet)
and 4.12% (13,274 square feet divided by 321,917 square feet), respectively.

     6.5 Payment to Mortgagee.  Landlord reserves the right to provide in
         --------------------                                                
any Mortgage given by it of the Property that some or all rents, issues, and
profits and all other amounts of every kind payable to the Landlord under this
Lease shall be paid directly to the Mortgagee for Landlord's account and Tenant
covenants and agrees that it will, after receipt by it of notice from Landlord
or Mortgagee designating such Mortgagee to whom payments are to be made by
Tenant, pay such amounts thereafter becoming due directly to such Mortgagee
until excused therefrom by notice from such Mortgagee.

     6.6 Right to Audit Records.  Tenant and Tenant's agents upon reasonable
         ----------------------                                               
prior notice may inspect Landlord's books and records with respect to taxes and
operating expenses of the Property as to which Tenant is obligated to pay
Additional Rent to Landlord pursuant to Section 6.2 and

                                       9
<PAGE>
 
Section 6.3, respectively, in accordance with the audit procedures set forth in
Exhibit C attached hereto and made a part hereof. If any such audit reveals that
---------
such taxes or operating expenses, as the case may be, were overstated, Landlord
shall reimburse Tenant within thirty (30) days after Landlord has been furnished
with a copy of such audit, the excess amount paid by Tenant to Landlord as a
consequence of such overstatement.

7.   UTILITIES AND LANDLORD'S SERVICES

     7.1 Electricity.  Tenant shall pay to Landlord monthly an amount
         -----------                                                   
reasonably estimated by Landlord to equal Tenant's Allocable Electricity Costs
for the electrical energy that Tenant utilizes for light and power in the
Demised Premises.  "Tenant's Allocable Electricity Costs" as used herein shall
be as determined in accordance with Exhibit D annexed hereto and made a part
                                    ---------                               
hereof.  Landlord shall furnish to Tenant quarterly a statement setting forth in
reasonable detail the particulars relating to Tenant's Allocable Electricity
Costs for the three months to which such a statement relates.  In the event the
estimated payments made by Tenant for said three (3) month period shall be less
than Tenant's Allocable Electricity Costs for said period as set forth in said
statement, Tenant shall promptly remit to Landlord the difference.  In the event
the estimated payments made by Tenant for said three (3) month period exceed
Tenant's Allocable Electricity Costs for said period as set forth in said
statement, such excess shall be refunded by Landlord. Landlord shall not be
liable in any way to Tenant for any failure or defect in the supply or character
of electrical energy furnished to the Demised Premises by reason of any
requirement, act or omission of the public utility serving the Building with
electricity unless due to the act or omission of Landlord. Tenant's use of
electrical energy in the Demised Premises shall not at any time exceed the
capacity of any of the electrical conductors and equipment in or otherwise
serving the Demised Premises.  In order to insure that such capacity is not
exceeded and to avert possible adverse affect upon the Building electrical
services, Tenant shall give notice to Landlord and obtain Landlord's prior
written consent whenever Tenant shall connect to the Building electrical
distribution system any fixtures, appliances or equipment.  If at any time the
electrical service to the Demised Premises is inadequate, in Tenant's reasonable
judgment, for the Permitted Use, Tenant shall have the right, subject to
Landlord's prior written consent which consent shall not be unreasonably
withheld or delayed, to install, at Tenant's sole cost and expense, any
additional feeders or risers reasonably necessary to supply Tenant's electrical
requirements in addition to those originally installed and all other equipment
proper and necessary in connection with such feeders, provided that such
additional feeders and risers are permissible under applicable laws and
insurance regulations and the installation of such feeders or risers will not
cause permanent damage or injury to the Building or cause or create a dangerous
condition or unreasonably interfere with other tenants of the Building.  Tenant
agrees that it will not make any alteration or material addition to the
electrical equipment and/or appliances in the Premises without the prior written
consent of Landlord in each instance first obtained (including, but not limited
to consent with respect to Tenant's contractor), which consent will not be
unreasonably withheld, and will promptly advise Landlord of any alteration or
addition to such electrical equipment and/or appliances.  Tenant, at Tenant's
expense, shall  purchase, install and replace all light fixtures, bulbs, tubes,
lamps, lenses, globes, ballasts and switches used in the Demised Premises.
Notwithstanding the foregoing, Landlord, at its election, may install at
Landlord's cost and

                                       10
<PAGE>
 
expense a separate meter for Tenant's electric usage in which event Tenant shall
thereafter obtain and pay for its electricity directly from the electric utility
servicing the Building.

     7.2 Water Charges.  Landlord shall furnish hot and cold water for
         -------------                                                  
ordinary cleaning, toilet, lavatory and drinking purposes to the extent required
to service facilities shown on the Plans approved by Landlord pursuant to
Section 4.3.  If Tenant requires, uses or consumes water for any purpose other
than for such purposes, Landlord may (i) assess a reasonable charge for the
additional water so used or consumed by Tenant or (ii) install a water meter and
thereby measure Tenant's water consumption for all purposes.  In the latter
event, Landlord shall pay the cost of the meter and the cost of installing any
equipment required in connection therewith, and Tenant shall keep said meter and
installation equipment in good working order and repair, and shall pay for water
consumed, as shown on said meter, together with the sewer charge based on said
meter charges, as and when bills are rendered.  On Tenant's default in making
such payment Landlord may pay such charges and collect the same from Tenant.

     7.3 Heat and Air Conditioning.  Landlord shall, through the equipment of
         -------------------------                                             
the Building furnish to and distribute in the Demised Premises heat and air
conditioning as normal seasonal changes may require on Business Days from 8:00
a.m. to 6:00 p.m. and on Saturdays (other than legal or recognized holidays as
defined in Article 1) from 8:00 a.m. to 12:00 noon when reasonably required for
the comfortable occupancy of the Demised Premises by Tenant.  Tenant agrees to
lower and close the blinds or drapes when necessary because of the sun's
position, whenever the air conditioning system is in operation, and to cooperate
fully with Landlord with regard to, and to abide by all the regulations and
requirements which Landlord may prescribe for the proper functioning and
protection of the heating and air conditioning system.

     7.4 Additional Heat, Cleaning and Air Conditioning Services.
         -------------------------------------------------------   

          (a)  The heating and air conditioning equipment serving the Demised
Premises shall be designed so as to permit Tenant to obtain heat and air
conditioning on days and at times other than as set forth in Section 7.3.
Tenant's use of such additional heat and air conditioning shall be gauged on the
meter referred to in Section 7.1 or otherwise reasonably estimated by Landlord,
and all charges in connection therewith shall be payable directly by Tenant.

          (b)  Tenant will pay to Landlord a reasonable charge for any extra
cleaning of the Premises required because of the carelessness or indifference
of Tenant or because of the nature of Tenant's business, or furnished by
Landlord at Tenant's request.  Landlord will endeavor to furnish such requested
extra cleaning service upon reasonable advance written notice from Tenant of its
requirements in that regard.

                                       11
<PAGE>
 
     7.5 Repairs and Other Services.  Except as otherwise provided in
         --------------------------                                       
Articles 16 and 18, and subject to Tenant's obligations in Article 12 and
elsewhere in this Lease, Landlord shall (a) keep and maintain the roof, exterior
walls, structural floor slabs and columns, windows (exclusive of glass
components), and common areas of the Building in as good condition and repair as
they are in on the Term Commencement Date, reasonable use and wear excepted, (b)
keep and maintain in workable condition the Building's sanitary, electrical,
heating, air conditioning and other systems, (c) provide cleaning services to
the Demised Premises and the common areas of the Building on Business Days
according to the cleaning specifications set forth in Exhibit E attached hereto
                                                      ---------                
and made a part hereof, (d) provide maintenance and snow removal for all
roadways, walkways and parking areas on the Property, (e) provide grounds
maintenance to all landscaped areas, and (f) employ a uniformed guard to be
stationed at the main entrance of the Building on an around-the-clock basis.
All expenses incurred by Landlord in connection with the foregoing repairs and
other services shall be included as part of operating expenses pursuant to
Section 6.3.

     7.6 Interruption or Curtailment of Services.  Landlord reserves the
         ---------------------------------------                              
right to interrupt, curtail, stop or suspend the furnishing of services and the
operation of any Building system, when necessary by reason of accident or
emergency, or of repairs, alterations, replacements or improvements in the
reasonable judgment of Landlord desirable or necessary to be made, or of
difficulty or inability in securing supplies or labor, or of strikes, or of any
other cause beyond the reasonable control of Landlord, whether such other cause
be similar or dissimilar to those hereinabove specifically mentioned, until said
cause has been removed, provided, however, that Landlord shall not interrupt
service to the dedicated electric riser referenced in Section 2.2 without
Tenant's prior consent, except in the event of casualty or an emergency.
Landlord shall have no responsibility or liability for any such interruption,
curtailment, stoppage, or suspension of services or systems, except that
Landlord shall exercise reasonable diligence to minimize inconvenience to Tenant
and to eliminate the cause of same and except that Landlord shall give
reasonable notice of such interruption, curtailment, stoppage or suspension
except when due to accident or emergency.

8.   CHANGES OR ALTERATIONS BY LANDLORD

     Landlord reserves the right, exercisable by itself or its nominee, at any
time and from time to time without the same constituting an actual or
constructive eviction and without incurring any liability to Tenant therefor or
otherwise affecting Tenant's obligations under this Lease, to make such changes,
alterations, additions, improvements, repairs or replacements in or to the
Building (including the Demised Premises) and the fixtures and equipment
thereof, as well as in or to the street entrances, halls, passages, elevators,
and stairways thereof, as it may deem necessary or desirable, and to change the
arrangement and/or location of entrances or passageways, doors and doorways, and
corridors, elevators, stairs, toilets, or other public parts of the Building,
provided, however, that there be no unreasonable obstruction of the right of
access to, or unreasonable interference with the use and enjoyment of, the
Demised Premises by Tenant, except that Landlord shall not be obligated to
employ labor at so-called "overtime" or other premium pay rates.  Nothing
contained in this Article shall be deemed to relieve Tenant of any duty,
obligation or liability of Tenant with respect to making or causing to be made
any repair, replacement or

                                       12
<PAGE>
 
improvement or complying with any law, order or requirement of any governmental
or other authority. Landlord reserves the right to from time to time change the
address of the Building. Neither this Lease nor any use by Tenant shall give
Tenant any right or easement or the use of any door or any passage or any
concourse connecting with any other building or to any public convenience, and
the use of such doors, passages and concourses and of such conveniences may be
regulated or discontinued at any time and from time to time by Landlord and
without affecting the obligation of Tenant hereunder or incurring any liability
to Tenant therefor.

9.   FIXTURES, EQUIPMENT AND IMPROVEMENTS - REMOVAL BY TENANT

     All fixtures, equipment, improvements and appurtenances "built into the
Demised Premises" prior to or during the Term, whether by Landlord at its
expense or at the expense of Tenant (either or both) or by Tenant shall be and
remain part of the Demised Premises and shall not be removed by Tenant at the
end of the Term unless otherwise expressly provided in this Lease.  For purposes
of this Lease, "built into the Demised Premises" shall mean that the applicable
fixtures, equipment, improvements and appurtenances are incorporated into the
Demised Premises to such a degree that the removal of the applicable item would
adversely affect the structure of the Demised Premises.  For all fixtures,
equipment, improvements and appurtenances that are installed in the Demised
Premises by Tenant, but are not "built into the Demised Premises", including,
but not limited to, all removable electric fixtures, signs, furniture, or trade
fixtures or business equipment, such items may be removed by Tenant upon the
condition that Tenant shall repair any damage to the Demised Premises or the
Building arising from such removal.

10.  ALTERATIONS AND IMPROVEMENTS BY TENANT; RESTORATION TO SHELL CONDITION

     (a) Tenant shall make no alterations, installations, removals, additions or
improvements in or to the Demised Premises without Landlord's prior written
consent and then only by contractors or mechanics approved by Landlord.  No
installations or other such work shall be undertaken or begun by Tenant until
Landlord has approved written plans and specifications therefor; and no
amendments or additions to such plans and specifications shall be made without
prior written consent of Landlord, which consent shall not be unreasonably
withheld or delayed. Any such work, alterations, decorations, installations,
removals, additions and improvements shall be done at the sole expense of Tenant
and at such times and in such manner as Landlord may from time to time
reasonably designate.

     (b) Landlord, at Landlord's option, by written notice thereof to Tenant
given at any time prior to the date which is sixty (60) days prior to the
Termination Date, may require Tenant, at Tenant's sole and expense, to restore
the Demised Premises to "shell condition", which may include, but shall not
limited to, removal of all data wiring, electric wiring, liebert units, the
halon system, the raised computer floor (to the extent located in the Demised
Premises), and removal of all batteries and equipment from the battery room.

                                       13
<PAGE>
 
11.  TENANT'S CONTRACTORS - MECHANICS' AND OTHER LIENS - STANDARD OF TENANT'S
     PERFORMANCE - COMPLIANCE WITH LAWS

     Whenever Tenant shall make any alterations, decoration, installations,
removals, additions or improvements or do any other work in or to the Demised
Premises, Tenant will strictly observe the following covenants and agreements:

          (a) In no event shall any material or equipment be incorporated in or
added to the Demised Premises in connection with any such alteration,
decoration, installation, addition or improvement which shall cause the Demised
Premises to be subject to any lien, charge, mortgage or other encumbrance of any
kind whatsoever.  Any mechanic's lien filed against the Demised Premises or the
Building for work claimed to have been done for, or materials claimed to have
been furnished to Tenant shall be discharged by Tenant within thirty (30) days
thereafter, at the expense of Tenant, by filing the bond required by law or
otherwise.  If Tenant fails so to discharge any lien, Landlord may do so at
Tenant's expense and Tenant shall reimburse Landlord for any expense or cost
incurred by Landlord in so doing within fifteen (15) days after rendition of a
bill therefor.

          (b) All installations or work done by Tenant under this or any other
Article of this Lease shall be at its own expense (unless expressly otherwise
provided) and shall at all times comply with (i) laws, rules, orders and
regulations of governmental authorities having jurisdiction thereof; (ii)
orders, rules and regulations of any Board of Fire Underwriters, or any other
body hereafter constituted exercising similar functions, and governing insurance
rating bureaus; (iii) plans and specifications prepared by and at the expense of
Tenant theretofore submitted to Landlord for its prior written approval.

          (c) Tenant shall procure all necessary permits before undertaking any
work in the Demised Premises; do all such work in a good and workmanlike manner,
employing materials of good quality and complying with all governmental
requirements and defend, save harmless, exonerate and indemnify Landlord from
all injury, loss or damage to any person or property occasioned by or growing
out of such work.

12.  REPAIRS AND SECURITY BY TENANT

     Tenant shall keep or cause to be kept all and singular the interior, non-
structural portions of the Demised Premises neat and clean and in such repair,
order and condition as the same are in on the Term Commencement Date or may be
put in during the term hereof, reasonable use and wear thereof, damage by fire
or by other insured casualty and repairs required to be made hereunder by
Landlord excepted.

     Tenant shall make, as and when needed as a result of misuse by, or neglect
or improper conduct (including without limitation the placement of any weight
exceeding the floor load) of Tenant or Tenant's servants, employees, agents,
invitees or licensees or otherwise, all repairs in

                                       14
<PAGE>
 
and about the interior, non-structural portions of the Demised Premises
necessary to preserve them in such repair, order and condition, which repairs
shall be in quality and class equal to the original work. Landlord may elect, at
the expense of Tenant, either pursuant to Section 15.3 or otherwise, to make any
such repairs or to repair any damage or injury to the Building or the Demised
Premises caused by moving property of Tenant in or out of the Building, or by
installation or removal of furniture or other property, or by misuse by, or
neglect or improper conduct of, Tenant or Tenant's servants, employees, agents
or licensees.

13.  INSURANCE, INDEMNIFICATION, EXONERATION AND EXCULPATION

     13.1 Insurance.  Tenant shall procure, keep in force and pay for (a)
          ---------                                                       
Comprehensive Public Liability Insurance indemnifying Landlord, any managing
agent designated by Landlord, Tenant and (whenever Landlord shall so request)
any Mortgagee against all claims and demands for injury to or death of persons
or damage to property which may be claimed to have occurred upon the Demised
Premises in the amounts which shall at the time Tenant and/or contractors enter
the Premises in accordance with Article 4 of this Lease be not less than Two
Hundred Thousand Dollars ($200,000) for property damage, One Million Dollars
($1,000,000) for injury or death of one person, and Two Million Dollars
($2,000,000) for injury or death of more than one person in a single accident,
and from time to time thereafter shall be not less than such higher amounts, if
procurable, as may be reasonably required by Landlord and are customarily
carried by responsible office tenants in the Greater Boston area, (b) insurance
covering any damage to the plate glass windows in or immediately about the
Demised Premises, in reasonable amounts to be established from time to time by
Landlord, and (c) so-called contents and improvements insurance adequately
insuring all property belonging to or removable by Tenant and situated in the
Demised Premises.

     13.2 Certificates of Insurance.  Such insurance shall be effected with
          -------------------------                                            
insurers authorized to do business in Massachusetts under valid and enforceable
policies, and such policies shall name Landlord, each Mortgagee, Tenant and any
additional parties designated by Landlord pursuant to Section 13.1 as the
insureds, as their respective interests appear.  Such insurance shall provide
that it shall not be cancelled without at least ten (10) days' prior written
notice to each insured named therein.  On or before the Term Commencement Date
and thereafter not less than fifteen (15) days prior to the expiration date of
each expiring policy, certificates of the policies provided for in Section 13.1
issued by the respective insurers, setting forth in full the provisions thereof
and issued by such insurers together with evidence satisfactory to Landlord of
the payment of all premiums for such policies, shall be delivered by Tenant to
Landlord and certificates as aforesaid of such policies shall, upon request of
Landlord, be delivered by Tenant to the holder of any mortgage affecting the
Demised Premises.

     13.3 General.  Tenant will save Landlord harmless, and will exonerate and
          -------                                                               
indemnify Landlord, from and against any and all claims, liabilities or
penalties asserted by or on behalf of any person, firm, corporation or public
authority:

          (a) On account of or based upon any injury to person, or loss of or
damage to property sustained or occurring on the Demised Premises on account of
or based upon the act,

                                       15
<PAGE>
 
omission, fault, negligence or misconduct of any person whomsoever (other than
as a consequence of the negligence or wilful misconduct of Landlord or its
agents or employees);

          (b) On account of or based upon any injury to person or loss of or
damage to property, sustained or occurring elsewhere (other than on the Demised
Premises) in or about the Building (and, in particular, without limiting the
generality of the foregoing on or about the elevators, stairways, public
corridors, sidewalks, concourses, arcades, malls, galleries, vehicular tunnels,
approaches, areaways, roof, or other appurtenances and facilities used in
connection with the Building or Demised Premises) arising out of the use or
occupancy of the Building or Demised Premises by the Tenant, or any person
claiming by, through or under Tenant;

          (c) On account of or based upon (including monies due on account of)
any work or thing whatsoever done (other than by Landlord or its contractors, or
agents or employees of either) in the Demised Premises during the Term of this
Lease and during the period of time, if any, prior to the Term Commencement Date
that Tenant may have been given access to the Demised Premises; and

          (d) On account of or resulting from the failure of Tenant to perform
and discharge any of its covenants and obligations under this Lease;

and, in respect of any of the foregoing items (a) - (d), from and against all
costs, expenses (including reasonable attorneys' fees), and liabilities incurred
in or in connection with any such claim, or any action or proceeding brought
thereon; and in case any action or proceeding be brought against Landlord by
reason of any such claim, Tenant upon notice from Landlord shall at Tenant's
expense resist or defend such action or proceeding and employ counsel therefor
reasonably satisfactory to Landlord, it being agreed that such counsel as may
act for insurance underwriters of Tenant engaged in such defense shall be deemed
satisfactory.

     13.4 Property of Tenant.  In addition to and not in limitation of the
          ------------------                                                  
foregoing, Tenant covenants and agrees that all merchandise, furniture, fixtures
and property of every kind, nature and description which may be in or upon the
Demised Premises or Building, in the public corridors, or on the sidewalks,
areaways and approaches adjacent thereto, during the term hereof, shall be at
the sole risk and hazard of Tenant, and that if the whole or any part thereof
shall be damaged, destroyed, stolen or removed from any cause or reason
whatsoever no part of said damage or loss shall be charged to, or borne by
Landlord.

     13.5 Bursting of Pipes, etc.  Landlord shall not be liable for any injury
          ----------------------                                                
or damage to persons or property resulting from fire, explosion, falling
plaster, steam, gas, electricity, electrical disturbance, water, rain or snow or
leaks from any part of the Building or from the pipes, appliances or plumbing
works or from the roof, street or sub-surface or from any other place or caused
by dampness or by any other cause of whatever nature, unless caused by or due to
the negligence of Landlord, its agents, servants or employees, and then only
after (i) notice to Landlord of the condition claimed to constitute negligence
an (ii) the expiration of a reasonable time after such notice has been received
by Landlord without such condition having been cured or

                                       16
<PAGE>
 
corrected; and in no event shall Landlord be liable for any loss, the risk of
which is compensated by Tenant's insurance; nor shall Landlord or its agents be
liable for any such damage caused by other tenants or persons in the Building or
caused by operations in construction of any private, public or quasi-public
work.

     13.6 Repairs and Alterations - No Diminution of Rental Value. Except as
          -------------------------------------------------------
otherwise provided in Articles 16 or 18, there shall be no allowance to Tenant
for diminution of rental value and no liability on the part of Landlord by
reason of inconvenience, annoyance or injury to Tenant arising from any repairs,
alterations, additions, replacements or improvements made by Landlord, Tenant or
others in or to any portion of the Building or Demised Premises, or in or to
fixtures, appurtenances, or equipment thereof, or for failure of Landlord or
others to make any repairs, alterations, additions or improvements in or to any
portion of the Building or of the Demised Premises, or in or to the fixtures,
appurtenances or equipment thereof provided, however, that any such work
performed by or on behalf of Landlord shall be subject to the provisions of
Section 15.2.

14.  ASSIGNMENT, MORTGAGING, SUBLETTING, ETC.

     14.1 Restrictions.  Tenant covenants and agrees that neither this Lease
          ------------                                                        
nor the term and estate hereby granted nor any interest herein or therein, will
be assigned, sublet, mortgaged, pledged, encumbered or otherwise transferred
(whether voluntarily or by operation of law) and that neither the Demised
Premises, nor any part thereof, will be encumbered in any manner by reason of
any act or omission on the part of Tenant, or used or occupied, or permitted to
be used or occupied, or utilized for any reason whatsoever, by anyone other than
Tenant, or for any use or purpose other than a stated in Article 1 without the
prior written consent of Landlord in every case.

     14.2 Requests to Assign or Sublet.  In connection with any request by
          ----------------------------                                      
Tenant for such consent to assign or sublet, Tenant shall submit to Landlord, in
writing, a statement containing the name of the proposed assignee or subtenant,
such information as to its financial responsibility and standing as Landlord may
reasonably require, and all of the terms and provisions upon which the proposed
assignment or subletting is to be made, and, unless the proposed area to be
assigned or sublet shall constitute an entire floor or floors, such statement
shall be accompanied by a floor plan delineating the proposed area to be
assigned or sublet.  In no event shall Tenant sublet the Demised Premises for a
rent which is less than the then fair market rent for new leases in the Building
as determined by Landlord in its sole reasonable discretion.  As long as Tenant
is not in default under any of the terms, covenants and conditions of this Lease
on Tenant's part to be observed and performed, Landlord shall not unreasonably
withhold or delay Landlord's prior consent to the assignment or subletting(s) by
Tenant of all or parts of the Demised Premises.  Each such subletting be for
undivided occupancy by the subtenant of that part of the Demised Premises
affected thereby for the use permitted under this Lease and at no time shall
there be more than three (3) occupants, including Tenant, within the Demised
Premises.  Landlord may, however, withhold such consent if, in Landlord's
reasonable judgment, the proposed assignee or subtenant is not engaged in a
business consistent with the character and dignity of the Building, or will
impose

                                       17
<PAGE>
 
any additional material burden upon Landlord in the operation of the Building
(to an extent greater than the burden to which Landlord would have been put if
Tenant continued to use, or used, such part of the Demised Premises for its own
purposes), or if Landlord has any other reasonable objections to the proposed
assignment or subletting. Any Colocation shall not be deemed an assignment or
subletting.

     14.3 Exceptions.  Notwithstanding the foregoing, Tenant may, without the
          ----------                                                           
requirement of obtaining Landlord's consent, assign this Lease or sublease any
portion of the Demised Premises to any entity which is (i) the parent, a
majority-owned subsidiary of Tenant, (ii) an entity under common control with
Tenant, (iii) an entity with which Tenant may merge or consolidate or to which
Tenant may sell all or substantially all of its assets as a going concern (such
entity with which Tenant may merge, consolidate or to which Tenant may sell all
or substantially all of its assets as aforesaid being hereinafter referred to as
a "Successor"), and (iv) an entity having a net worth in excess of $20,000,000
at the time of such assignment or subletting (an "Unrelated Transferee"),
provided that, simultaneously with any such assignment, Tenant shall deliver to
Landlord an agreement in form and substance reasonably satisfactory to Landlord
which contains an appropriate covenant of assumption by such assignee; and
provided further that in the case of any such assignment or sublease to a
Successor, Tenant shall have submitted to Landlord prior thereto financial
statements or other materials reasonably satisfactory to Landlord evidencing
that such Successor has financial resources comparable to that of Tenant as of
the time of such assignment or sublease.  In addition, in the case of an
assignment or subletting to an Unrelated Transferee, Tenant shall have submitted
to Landlord prior thereto financial statements or other materials reasonably
satisfactory to Landlord evidencing that such Unrelated Transferee has a net
worth in excess of $20,000,000 at the time of such assignment or subletting.

     14.4 Excess Rent.  Except for subleases or assignments pursuant to
          -----------                                                    
Section 14.3, if the rent received by Tenant on account of a sublease of all or
any portion of the Demised Premises exceeds the Yearly Fixed Rent and Additional
Rent, allocated to the space subject to the sublease in the proportion of the
area of such space to the area of the entire Demised Premises, Tenant shall pay
to Landlord such excess, monthly as received by Tenant.

     14.5 Recapture.  Notwithstanding the foregoing provisions of this Article
          ---------                                                            
and except in the cases of subleases or assignments pursuant to Section 14.3:
(1) in the event Tenant proposes to assign or sublet all of the Demised
Premises, Landlord, at Landlord's option, may give to Tenant, within ten (10)
business days after the submission by Tenant to Landlord of the statement
required to be submitted in connection with such assignment or subletting, or,
if Tenant so requests, within ten (10) business days after Tenant notifies
Landlord that Tenant wishes to undertake such assignment or subletting, but has
not yet procured a proposed assignee or subtenant, a notice terminating this
Lease on the date (referred to as the "Earlier Termination Date") immediately
prior to the proposed commencement date of the term of the proposed assignment
or subletting, as set forth in such statement, and, in the event such notice is
given, this Lease and the Term shall come to an end and expire on the Earlier
Termination Date with the same effect as if it were the date originally fixed in
this Lease for the end of the Term of this Lease, and the Rent shall be
apportioned as of said Earlier Termination Date and any prepaid portion of Rent
for any period

                                       18
<PAGE>
 
after such date shall be refunded by Landlord to Tenant, provided, however, that
in the event Landlord shall so elect to terminate this Lease, Tenant, upon
written notice to Landlord given within ten (10) days of receipt by Tenant of
Landlord's notice of termination, may elect to negate such termination by
declaring its intent not to proceed with such assignment or subletting; or (2)
in the event Tenant proposes to assign or sublet in the aggregate in excess of
fifty percent (50%) of the original square footage of the Demised Premises,
Landlord, at Landlord's option, may give to Tenant, within thirty (30) days
after the submission by Tenant to Landlord of the statement required to be
submitted in connection with such proposed assignment or subletting, a notice
electing to eliminate such portion of the Demised Premises (said portion is
referred to as the "Eliminated Space") from the Demised Premises during the
period (referred to as the "Elimination Period") commencing on the date
(referred to as the "Elimination Date") immediately prior to the proposed
commencement date of the term of the proposed assignment or subletting, as set
forth in such statement, and ending on the proposed expiration date of the term
of the proposed assignment or subletting, as set forth in such statement, and in
the event such notice is given (i) the Eliminated Space shall be eliminated from
the Demised Premises during the Elimination Period; (ii) Tenant shall surrender
the Eliminated Space to Landlord on or prior to the Elimination Date in the same
manner as if said Date were the date originally fixed in this Lease for the end
of the Term of this Lease; (iii) if the Eliminated Space shall constitute less
than an entire floor, Landlord, at Landlord's expense, shall have the right to
make any alterations and installations in the Demised Premises required, in
Landlord's judgment, reasonably exercised, to make the Eliminated Space a self-
contained rental unit with access through corridors to the elevators and core
toilets serving the Eliminated Space, and if the Demised Premises shall contain
any core toilets or any corridors (including any corridors proposed to be
constructed by Landlord pursuant to this subdivision (iii) providing access from
the Eliminated Space to the core area), Landlord and any tenant or other
occupant of the Eliminated Space shall have the right to use such toilets and
corridors in common with Tenant and any other permitted occupants of the Demised
Premises, and the right to install signs and directional indicators in or about
such corridors indicating the name and location of such tenant or other
occupant; (iv) during the Elimination Period, the Yearly Fixed Rent shall be
reduced in the proportion which the area of the Eliminated Space bears to the
total area of the Demised Premises immediately prior to the Elimination Date
(including an equitable portion of the area of any corridors referred to in
subdivision (iii) of this sentence as part of the area of the Eliminated Space
for the purpose of computing such reduction), and any prepaid Rent for any
period after the Elimination Date allocable to the Eliminated Space shall be
refunded by Landlord to Tenant; (v) there shall be an equitable apportionment of
any Additional Rent Payable pursuant to Article 6 for the relevant fiscal and
calendar years in which said Elimination Date shall occur; and (vi) if the
Elimination Period shall end prior to the date originally fixed in this Lease
for the end of the Term of this Lease, the Eliminated Space, in its then
existing condition, shall be deemed restored to and once again a part of the
Demised Premises subject to the provisions of this Lease as if said elimination
had not occurred during the period (referred to as the "Restoration Period")
commencing on the date next following the expiration of the Elimination Period
and ending on the date originally fixed in this Lease for the end of the Term of
this Lease, except in the event that Landlord is unable to give Tenant
possession of the Eliminated Space at the expiration of the Elimination Period
by reason of the holding over or retention of possession of any tenant or other
occupant, in which event (x) the Restoration Period shall not commence, and 

                                       19
<PAGE>
 
the Eliminated Space shall not be deemed restored to or a part of the Demised
Premises, until the date upon which Landlord shall give Tenant possession of
such Space free of occupancies, provided Landlord shall use reasonable efforts
to return the Eliminated Space to Tenant at the expiration of the Elimination
Period, (y) neither the date fixed in this Lease for the end of the Term of the
Lease, nor the validity of this Lease shall be affected and (z) Tenant waives
any right to recover any damages which may result from the failure of Landlord
to deliver possession of the Eliminated Space at the end of the Elimination
Period excepting only those resulting from Landlord's gross negligence or
willful misconduct.

     14.6 Further Documentation.  At the request of Landlord, Tenant shall
          ---------------------                                             
execute and deliver an instrument or instruments, in form satisfactory to
Landlord, setting forth any modifications to this Lease contemplated in or
resulting from the operation of the foregoing provisions of this paragraph;
however, neither Landlord's failure to request any such instrument nor Tenant's
failure to execute or deliver any such instrument shall vitiate the effect of
the foregoing provisions of this Article.

     14.7 General.
          -------   

          (a)  The failure by Landlord to exercise its option under this
paragraph with respect to any subletting shall not be deemed a waiver of such
option with respect to any extension or any subsequent subletting of the
premises affected thereby.  Tenant shall reimburse Landlord promptly, as
Additional Rent, for reasonable legal and other expense incurred by Landlord in
connection with any request by Tenant for any consent required under the
provisions of this Article.

          (b)  Except for Colocation and except as set forth in Section 14.7(e)
below, it is specifically understood and agreed that neither Tenant nor any
other person having an interest in the possession, use, occupancy or utilization
of the Demised Premises shall enter into any sublease, license, concession or
other agreement (or renewals of any of the foregoing) for use, occupancy or
utilization of space in the Demised Premises which provides for rental or other
payment for such use, occupancy or utilization based, in whole or in part, on
the net income or profits derived by any person or entity from the space leased,
used, occupied or utilized (other than an amount based on a fixed percentage or
percentages of receipts or sales).  Any such purported sublease or other
agreement shall be absolutely void and ineffective as a conveyance of any right
or interest in the possession, use, occupancy, or utilization of any part of the
Demised Premises.

                                       20
<PAGE>
 
          (c)  The listing of any name other than that of Tenant, whether on the
doors of the Demised Premises or on the Building directory, or otherwise, shall
not operate to vest any right or interest in this Lease or in the Demised
Premises or be deemed to  be the written consent of Landlord mentioned in this
Article, it being expressly understood that any such listing is a privilege
extended by Landlord revocable at will by written notice to Tenant.

          (d)  If this Lease be assigned, or if the Demised Premises or any part
thereof shall be sublet or occupied by anybody other than Tenant, Landlord may
at any time and from time to time, collect rent and other charges from the
assignee, subtenant or occupant and apply the aggregate amount collected to the
Rent and other charges herein reserved, but no such assignment or collection
shall be deemed a waiver of this covenant, or the acceptance of the assignee,
subtenant or occupant as a tenant, or a release of Tenant from the further
performance by Tenant of covenants on the part of Tenant herein contained.
Landlord shall notify Tenant of any amounts so collected by Landlord directly
from such subtenant or assignee.  The consent by Landlord to an assignment or
subletting or occupancy shall not in any way be construed to relieve Tenant from
obtaining the express consent in writing of Landlord to any further assignment
or subletting or occupancy.

          (e)  Landlord acknowledges that Tenant has agreed to permit the
preceding tenant of the Demised Premises, XCOM Technologies, Inc. ("XCOM"), to
continue to operate and maintain certain telecommunications equipment in the
Demised Premises (and such other Telecommunications Equipment and Cabling that
currently exists in the Building and was heretofore used by such XCOM) after the
Commencement Date in order to permit said tenant a sufficient period of time to
relocate itself to a new location outside the Building.  Landlord agrees that
such use shall not be deemed an assignment or subletting.  Tenant agrees that
Landlord shall not be liable in any manner to Tenant or any other party as a
consequence of such arrangement between Tenant and XCOM and that such
arrangement shall in no manner constitute a breach or give rise to any claims
under Article 22 hereof and/or any other provision of this Lease.

15.  MISCELLANEOUS COVENANTS

     15.1 Rules and Regulations. Tenant and Tenant's servants, employees, and
          ---------------------
agents will faithfully observe such Rules and Regulations as are attached hereto
as Exhibit F and made a part hereof or as Landlord hereafter at any time or from
   ---------
time to time may make and may communicate in writing to Tenant and which in the
reasonable judgment of Landlord shall be necessary for the reputation, safety,
care or appearance of the Property, or the preservation of good order therein,
or the operation or maintenance of the Property, or the equipment thereof, or
the comfort of tenants or others in the Building, provided, however, that in the
case of any conflict between the provisions of this Lease and any such Rules and
Regulations, the provisions of this Lease shall control, and provided further
that nothing contained in this Lease shall be construed to impose upon Landlord
any duty or obligation to enforce such Rules and Regulations or the terms,
covenants or conditions in any other lease as against any other tenant and
Landlord shall not be liable to Tenant for violation of the same by any other
tenant, its servants, employees, agents, visitors, invitees or licensees.

                                       21
<PAGE>
 
     15.2 Access to Premises - Shoring.  Tenant shall:  (i) permit Landlord to
          ----------------------------                                          
erect, use and maintain pipes, ducts and conduits in and through the Demised
Premises, provided the same do not materially reduce the floor area or
materially adversely affect the appearance thereof; (ii) permit the Landlord and
any Mortgagee of the Building or the Building and Land or of the interest of
Landlord therein, and any lessor under any ground or underlying lease, and their
representatives, to have free and unrestricted access to and to enter upon the
Demised Premises at all reasonable hours for the purposes of inspection or of
making  repairs, replacements or improvements in or to the Demised Premises or
the Building or equipment (including, without limitation, sanitary, electrical,
heating, air conditioning or other systems) or of complying with all laws,
orders and requirements of governmental or other authority or of exercising any
right reserved to Landlord by this Lease (including the right during the
progress of any such repairs, replacements or improvements or while performing
work and furnishing materials in connection with compliance with any such laws,
orders or requirements to take upon or through, or to keep and store within, the
Demised Premises all necessary materials, tools and equipment); and (iii) permit
Landlord, at reasonable times, to show the Demised Premises during ordinary
business hours to any Mortgagee, ground lessor, prospective purchaser,
prospective mortgagee, or prospective assignee of any mortgage, of the Building
or of the Building and the Land or of the interest of Landlord therein, and
during the period of twelve months next preceding the Termination Date to any
person contemplating the leasing of the Demised Premises or any part thereof.
If during the last month of the Term, Tenant shall have removed all of Tenant's
property therefrom, Landlord may immediately enter and alter, renovate and
redecorate the Demised Premises, without elimination or abatement of rent, or
incurring liability to Tenant for any compensation, and such acts shall have no
effect upon this Lease.  If Tenant shall not be personally present to open and
permit any entry into the Demised Premises at any time when for any reason an
entry therein shall be necessary or permissible, Landlord or Landlord's agents
must nevertheless be able to gain such entry by contacting a responsible
representative of Tenant, whose name, address and telephone number shall be
furnished by Tenant.  Provided that Landlord shall incur no additional expense
thereby, Landlord shall exercise its rights of access to the Demised Premises
permitted under any of the terms and provisions of this Lease in such manner as
to minimize to the extent practicable interference with Tenant's use and
occupation of the Demised Premises, and upon reasonable prior notice, except in
the case of an emergency or where such notice is not reasonably practical.  If
an excavation shall be made upon land adjacent to the Demised Premises or shall
be authorized to be made, Tenant shall afford, to the causing or authorized to
cause such excavation, license to enter upon the Demised Premises for the
purpose of doing such work as said person shall deem necessary to preserve the
Building from injury or damage and to support the same by proper foundations
without any claim for damage or indemnity against Landlord, or diminution or
abatement of Rent.

     15.3 Accidents to Sanitary and Other Systems.  Tenant shall give to
          ---------------------------------------                              
Landlord prompt notice of any fire or accident in the Demised Premises or in the
Building and of any damage to, or defective condition in, any part or
appurtenance of the Building's sanitary, electrical, heating and air
conditioning or other systems located in, or passing through, the Demised
Premises, and the damage or defective condition shall be remedied  by Landlord
with reasonable diligence, but if 

                                       22
<PAGE>
 
such damage or defective condition was caused by Tenant or by the employees,
licensees, or invitees of Tenant, the cost to remedy the same shall be paid by
Tenant. Tenant shall not be entitled to claim any eviction from the Demised
Premises or any damages arising from any such damage or defect unless the same
(i) shall have been occasioned by the negligence of Landlord, its agents,
servants or employees and (ii) shall not, after notice to Landlord of the
condition claimed to constitute negligence, have been cured or corrected within
a reasonable time after such notice has been received by Landlord; and in case
of a claim of eviction unless such damage or defective condition shall have
rendered the Demised Premises untenantable and they shall not have been made
tenantable by Landlord within a reasonable time. Landlord agrees to diligently
work to correct such damage or defect, except as otherwise provided herein.

     15.4 Signs, Blinds and Drapes. Tenant shall not place any signs on the
          ------------------------
exterior of the Building or on or in any window, public corridor or door visible
from the exterior of the Demised Premises. No blinds may be put on or in any
window nor may any Building drapes or blinds be removed by Tenant. Tenant may
hang its own drapes, provided that they shall not, without the prior written
approval of Landlord, in any way interfere with any Building drapery or blinds
or be visible from the exterior of the Building.

     15.5 Estoppel Certificate. Either party shall at any time and from time to
          --------------------
time upon not less than twenty (20) days' prior notice by Landlord to Tenant or
by a Mortgagee to Tenant, or by Tenant to Landlord, as the case may be, execute,
acknowledge and deliver to the party making such request a statement in writing
certifying that this Lease is unmodified and in full force and effect (or if
there have been modifications, that the same is in full force and effect as
modified and stating the modifications) and the dates to which Rent has been
paid in advance, if any, an stating whether or not to the best knowledge of the
signer of such certificate Landlord is in default in performance of any
covenant, agreement, term, provisions or condition contained in this Lease and,
if so, specifying each such default of which the signer may have knowledge, it
being intended that any such statement delivered pursuant hereto may be relied
upon by any prospective purchaser of the Building or of the Building and the
Land or of the interest of Landlord therein, any Mortgagee or prospective
Mortgagee thereof, any lessor or prospective lessor thereof, any lessee or
prospective lessee thereof, or any prospective assignee of any Mortgage. The
form of any such estoppel certificate requested by a Mortgagee shall be
satisfactory to such Mortgagee. Notwithstanding anything contained in this Lease
to the contrary, the provisions of this Section 15.5 shall be inapplicable to
Teachers Insurance and Annuity Association in the event said entity shall
succeed to the rights of Landlord hereunder.

     15.6 Prohibited Items. Tenant shall not bring or permit to be brought or
          ----------------
kept in or on the Demised Premises or elsewhere in the Building any hazardous,
inflammable, combustible or explosive fluid, material, chemical or substance
(except such as are related to Tenant's use of the Demised Premises, provided
that the same are stored and handled in a proper fashion consistent with all
applicable legal standards).


     15.7 Requirements of Law; Fines and Penalties. Tenant at its sole expense
          ----------------------------------------
shall comply 

                                       23
<PAGE>
 
with all laws, rules, orders and regulations of Federal, State, County and
Municipal Authorities and with any direction of any public officer or officers,
pursuant to law, which shall impose any duty upon Landlord or Tenant with
respect to and arising out of Tenant's use or occupancy of the Demised Premises.
Tenant shall reimburse and compensate Landlord for all expenditures made by, or
damages or fines sustained or incurred by, Landlord due to nonperformance or
noncompliance with or breach or failure to observe any term, covenant or
condition of this Lease upon Tenant's part to be kept, observed, performed or
complied with. If Tenant receives notice of any violation of law, ordinance,
order or regulation applicable to the Demised Premises, it shall give prompt
notice thereof to Landlord.

     15.8 Tenant's Acts - Effect on Insurance. Except for permitted uses, Tenant
          -----------------------------------
shall not do or permit to be done any act or thing upon the Demised Premises or
elsewhere in the Building which will invalidate or be in conflict with any
insurance policies covering the Building and the fixtures and property therein
and shall not do, or permit to be done, any act or thing upon the Demised
Premises which shall subject Landlord to any liability or responsibility for
injury to any person or persons or to property by reason of any business or
operation being conducted on said Demised Premises or for any other reason.
Tenant at its own expense shall comply with all rules, orders, regulations or
requirements of the Board of Fire Underwriters or any other similar body having
jurisdiction, and shall not (i) do, or permit anything to be done, in or upon
the Demised Premises, or bring or keep anything therein, except as now or
hereafter permitted by the Fire Department, Board of Underwriters, Fire
Insurance Rating Organization, or other authority having jurisdiction, and then
only in such quantity and manner of storage as will not increase the rate for
any insurance applicable to the Building, or (ii) use the Demised Premises in
manner which shall increase such insurance rates on the Building or on property
located therein, over that applicable when Tenant first took occupancy of the
Demised Premises hereunder. If by reason of failure of Tenant to comply with the
provisions hereof the insurance rate applicable to any policy of insurance shall
at any time thereafter be higher than it otherwise would be, then Tenant shall
reimburse Landlord for that part of any insurance premiums thereafter paid by
Landlord, which shall have been charged because of such failure by Tenant.

     15.9 Hazardous Materials. Tenant shall not (either with or without
          -------------------
negligence) cause or permit the escape, disposal or release of any biologically
or chemically active or other hazardous substances, or materials. Tenant shall
not allow the storage or use of such substances or materials in any manner not
sanctioned by law or by the highest standards prevailing in the industry for the
storage and use of such substances or materials, nor allow to be brought onto
the Property any such materials or substances except to use in the ordinary
course of Tenant's business, and then only after written notice is given to
Landlord of the identity of such substances or materials. Without limitation,
hazardous substances and materials shall include those described in the
Comprehensive Environmental Response, Compensation and Liability Act of 1980, as
amended, 42 U.S.C. Section 9601 et seq., any applicable state or local laws and
the regulations adopted under these acts. If any lender or governmental agency
shall ever require testing to ascertain whether or not there has been any
release of hazardous materials, then the reasonable costs thereof shall be
reimbursed by Tenant to Landlord upon demand as additional charges if such
requirement applies to the Demised Premises. In addition, Tenant shall execute
affidavits, representations and 

                                       24
<PAGE>
 
the like from time to time at Landlord's request concerning Tenant's best
knowledge and belief regarding the presence of hazardous substances or materials
on the Demised Premises. In all events, Tenant shall indemnify Landlord in the
manner elsewhere provided in this Lease from any release of hazardous materials
on the Demised Premises occurring while Tenant is in possession, or elsewhere if
caused by Tenant or persons acting under Tenant. The within covenants shall
survive the expiration or earlier termination of the Term of this Lease.

     15.10 Miscellaneous. Tenant shall not suffer or permit the Demised Premises
           -------------
or any fixtures, equipment or utilities therein or serving the same, to be
overloaded, damaged or defaced, nor permit any hole to be drilled or made in any
part thereof.

16.  DAMAGE BY FIRE, ETC.

     In the event of loss of, or damage to, the Demised Premises or the Building
by fire or other casualty, the rights and obligations of the parties hereto
shall be as follows:

          (a)  If the Demised Premises, or any part thereof, shall be damaged by
fire or other casualty, Tenant shall give prompt notice thereof to Landlord, and
Landlord, upon receiving such notice, shall proceed promptly and with due
diligence, subject to unavoidable delays, to repair, or cause to be repaired,
such damage.  If the Demised Premises or any part thereof shall be rendered
untenantable by reason of such damage, whether to the Demised Premises or to the
Building, the Yearly Fixed Rent and Additional Rent shall proportionately abate
for the period from the date of such damage to the date when such damage shall
have been repaired.

          (b)  If, as a result of fire or other casualty, the whole or a
substantial portion of the Building is rendered untenantable, Landlord, within
ninety (90) days from the date of such fire or other casualty, may terminate
this Lease by notice to Tenant, specifying a date not less than twenty (20) nor
more than forty (40) days after the giving of such notice on which the Term of
this Lease shall terminate.  If Landlord does not so elect to terminate this
Lease, then Landlord shall proceed with diligence to repair the damage to the
Demised Premises and all facilities serving the same, if any, which shall have
occurred, and the Yearly Fixed Rent and Additional Rent shall meanwhile
proportionately abate, all as provided in Paragraph (a) of this Section.
However, if, in the event of any damage by fire or other casualty, such damage
is not repaired and the Demised Premises and all facilities serving the same
restored to substantially the same condition as they were prior to such damage
within six (6) months from the date of such damage, Tenant within thirty (30)
days from the expiration of such six (6) month period, may terminate this Lease
by notice to Landlord, specifying a date not more than sixty (60) days after the
giving of such notice on which the term of this Lease shall terminate.

          (c)  If the Demised Premises shall be rendered untenantable by fire or
other casualty during the last two (2) years of the Term of this Lease, Landlord
may terminate this Lease effective as of the date of such fire or other casualty
upon notice to Tenant given within thirty (30) days after such fire or other
casualty.

                                       25
<PAGE>
 
          (d)  Landlord shall not be required to repair or replace any of
Tenant's business machinery, equipment, cabinet  work, furniture, personal
property or other installations (which shall, however, be substantially restored
by Tenant within ninety (90) days after Landlord shall have completed any repair
or restoration required under the terms of this Article), and no damages,
compensation or claim shall be payable by Landlord for inconvenience, loss of
business or annoyance arising from any repair or restoration of any portion of
the Demised Premises or of the Building.

          (e)  The provisions of this Article shall be considered an express
agreement governing any instance of damage or destruction of the Building or the
Demised Premises by fire or other casualty, and any law now or hereafter in
force providing for such a contingency in the absence of express agreement shall
have no application.

          (f)  In the event of any termination of this Lease pursuant to this
Article, the Term of this Lease shall expire as of the effective termination
date as fully and completely as if such date were the date herein originally
scheduled as the Termination Date.  Tenant shall have access to the Demised
Premises for a period of thirty (30) days after the date of termination in order
to remove Tenant's personal property.

          (g)  Landlord's Architect's certificate, given in good faith, shall be
deemed conclusive of the statements therein contained and binding upon Tenant
with respect to the performance and completion of any repair or restoration work
undertaken by Landlord pursuant to this Article or Article 18.  Any minor or
insubstantial details of construction or mechanical adjustments which remain to
be done after the delivery of said certificate shall be handled on a punch list
basis and thereafter promptly completed by Landlord.

17.  WAIVER OF SUBROGATION

     In any case in which Tenant shall be obligated under any provision of this
Lease to pay to Landlord any loss, cost, damage, liability, or expense suffered
or incurred by Landlord, Landlord shall allow to Tenant as an offset against the
amount thereof the net proceeds of any insurance collected by Landlord for or on
account of such loss, cost, damage, liability or expense, provided that the
allowance of such offset does not invalidate or prejudice the policy or policies
under which such proceeds were payable.

     In any case in which Landlord shall be obligated under any provision of
this Lease to pay to Tenant any loss, cost, damage, liability or expense
suffered or incurred by Tenant, Tenant shall allow to Landlord as an offset
against the amount thereof (i) the net proceeds of any insurance collected by
Tenant for or on account of such loss, cost, damage, liability, or expense,
provided that the allowance of such offset does not invalidate the policy or
policies under which such proceeds were payable and  (ii) if such loss, cost,
damage, liability or expense shall have been caused by a peril against which
Tenant has agreed to procure insurance coverage under the terms of this Lease,
the amount of such insurance coverage, whether or not actually procured by
Tenant.

                                       26
<PAGE>
 
     The parties hereto shall each endeavor to procure an appropriate clause in,
or endorsement on, any fire or extended coverage insurance policy covering the
Demised Premises and the Building and personal property, fixtures and equipment
located thereon or therein, pursuant to which the insurance companies waive
subrogation or consent to a waiver of right of recovery, and having obtained
such clauses and/or endorsements of waiver of subrogation or consent to a waiver
of right of recovery each party hereby agrees that it will not make any claim
against or seek to recover from the other for any loss or damage to its property
or the property of others resulting from fire or other perils covered by such
fire and extended coverage insurance; provided, however, that the release,
discharge, exoneration and covenant not to sue herein contained shall be limited
by the terms and provisions of the waiver of subrogation clauses and/or
endorsements or clauses and/or endorsements consenting to a waiver of right of
recovery and shall be co-extensive therewith.  If either party may obtain such
clause or endorsement only upon payment of an additional premium, such party
shall promptly so advise the other party and shall be under no obligation to
obtain such clause or endorsement unless such other party pays the premium.

18.  CONDEMNATION - EMINENT DOMAIN

     In the event that the whole or any material part of the Building shall be
taken or appropriated by eminent domain or shall be condemned for any public or
quasi-public use, or (by virtue of any such taking, appropriation or
condemnation) shall suffer any damage (direct, indirect or consequential) for
which Landlord or Tenant shall be entitled to compensation then (and in any such
event) this Lease and the Term hereof may be terminated at the election of
Landlord by a notice in writing of its election so to terminate which shall be
given by the Landlord to Tenant within sixty (60) days following the date on
which Landlord shall have received notice of such taking, appropriation or
condemnation.  In the event that more than twenty-five percent (25%) of the
floor area of the Demised Premises shall be so taken, appropriated or condemned,
then (and in any such event) this Lease and the Term hereof may be terminated at
the election of Tenant by a notice in writing of its election so to terminate
which shall be given by Tenant to Landlord within sixty (60) days following the
date on which Tenant shall have received notice of such taking, appropriation or
condemnation.

     Upon the giving of any such notice of termination (either by Landlord or
Tenant) this Lease and the Term hereof shall terminate on or retroactively as of
the date on which Tenant  shall be required to vacate any part of the Demised
Premises or shall be deprived of a substantial part of the means of access
thereto, provided, however, that Landlord may in Landlord's notice elect to
terminate this Lease and the Term hereof retroactively as of the date on which
such taking, appropriation or condemnation became legally effective.  In the
event of any such termination, this Lease and the Term hereof shall expire as of
the effective termination date as fully and completely as if such date were the
date herein originally scheduled as the Termination Date.  If neither party
(having the right so to do) elects to terminate Landlord will, with reasonable
diligence and at Landlord's expense, restore the remainder of the Demised
Premises, or the remainder of the means of access, as nearly as practicably may
be to the same condition as obtained prior to such taking, appropriation or
condemnation in which event (i) a just proportion of the Yearly Fixed Rent and
Additional Rent, according to the nature and extent of the taking, appropriation
or 

                                       27
<PAGE>
 
condemnation and the resulting permanent injury to the Demised Premises and the
means of access thereto, shall be permanently abated, and (ii) a just proportion
of the remainder of the Yearly Fixed Rent and Additional Rent, according to the
nature and extent of the taking, appropriation or condemnation and the resultant
injury sustained by the Premises and the means of access thereto, shall be
abated until what remains of the Premises and the means of access thereto shall
have been restored as fully as may be for permanent use and occupation by Tenant
hereunder. Except for any award specifically reimbursing Tenant for moving or
relocation expenses and Tenant's removable fixtures and equipment, there are
expressly reserved to Landlord all rights to compensation and damages created,
accrued or accruing by reason of any such taking, appropriation or condemnation,
in implementation and in confirmation of which Tenant does hereby acknowledge
that Landlord shall be entitled to receive and retain all such compensation and
damages, grants to Landlord all and whatever rights (if any) Tenant may have to
such compensation and damages, and agrees to execute and deliver all and
whatever further instruments of assignment as Landlord may from time to time
request. In the event of any taking of the Demised Premises or any part thereof
for temporary use, (i) this Lease shall be and remain unaffected thereby, and
(ii) Tenant shall be entitled to receive for itself any award made for such use,
provided, that if any taking is for a period extending beyond the Term of this
Lease, such award shall be apportioned between Landlord and Tenant as of the
Termination Date.

19.  DEFAULT

     19.1 Conditions of Limitation - Re-entry - Termination. This Lease and the
          -------------------------------------------------
herein term and estate are upon the condition that if (a) Tenant shall neglect
or fail to perform or observe any of the Tenant's covenants herein, including
(without limitation) the covenants with regard to the payment of Rent on or
before ten (10) days of its due date; or (b) Tenant shall be involved in
financial difficulties as evidenced by an admission in writing by Tenant of
Tenant's inability to pay its debts generally as they become due, or by the
making or offering to make a composition of its debts with its creditors; or (c)
Tenant shall make an assignment or trust mortgage, or other conveyance or
transfer of like nature, of all or a substantial part of its property for the
benefit of its creditors, or (d) the leasehold hereby created shall be taken on
execution or by other process of law and shall not be revested in Tenant within
sixty (60) days thereafter; or (e) a receiver, sequester, trustee or similar
officer shall be appointed by a court of competent jurisdiction to take charge
of all or a substantial part of Tenant's property and such appointment shall not
be vacated within sixty (60) days or (f) any proceeding shall be instituted by
or against Tenant pursuant to any of the provisions of any Act of Congress or
State law relating to bankruptcy, reorganization, arrangements, compositions or
other relief from creditors, and, in the case of any such proceeding instituted
against it, if Tenant shall fail to have such proceeding dismissed within thirty
(30) days or if Tenant is adjudged bankrupt or insolvent as a result of any such
proceeding; or (g) any event shall occur or any contingency shall arise whereby
this Lease, or the term and estate thereby created would (by operation of law or
otherwise) devolve upon or pass to any person, firm or corporation other than
Tenant, except as expressly permitted under Article 14 hereof; or (h) Tenant
shall vacate all or substantially all of the Demised Premises then, and in any
such event (except as hereinafter in Article 19.2 otherwise provided) Landlord
may, in a manner consistent with applicable law, immediately or at any time
thereafter declare this Lease terminated by notice 

                                       28
<PAGE>
 
to Tenant or, without further demand or notice, enter into and upon the Demised
Premises (or any part thereof in the name of the whole), and in either such case
(and without prejudice to any remedies which might otherwise be available for
arrears of rent or other charges due hereunder or preceding breach of covenant
and without prejudice to Tenant's liability for damages as hereinafter stated),
this Lease shall terminate. The words "re-entry" and "re-enter" as used in this
Lease are not restricted to their technical legal meaning.

     19.2 Damages - Assignment for Benefit of Creditors. For the more effectual
          ---------------------------------------------
securing by Landlord of the rent and other charge and payments reserved
hereunder, it is agreed as a further condition of this Lease that if at any time
Tenant shall make an assignment of its property for the benefit of its creditors
under the terms of which the debts provable by its creditors shall be debts
provable against the estate of insolvent debtors either under the laws of the
Commonwealth of Massachusetts or under some law or laws other than the
Bankruptcy Code as now or hereafter enacted, then and in any such case the same
shall constitute a breach of this Lease, and the term and estate hereby created
shall terminate ipso facto, without entry or other action by Landlord; and
notwithstanding any other provisions of this Lease Landlord shall forthwith upon
such termination, without prejudice to any remedies which might otherwise be
available for arrears of rent or other charges due hereunder or preceding breach
of this Lease, be ipso facto entitled to recover as liquidated damages the sum
of (a) the amount by which, at the time of such termination of this Lease, (i)
the aggregate of the Rent projected over the period commencing with such
termination and ending with the Termination Date stated in Article 1 exceeds
(ii) the aggregate projected rental value of the Demised Premises for such
period and (b) (in view of the uncertainty of prompt re-letting and the expense
entailed in re-letting the Demised Premises) an amount equal to the Rent payable
for and in respect of the calendar year next preceding the date of termination,
as aforesaid. Upon such termination Landlord, may immediately or at any time
thereafter, without demand or notice, enter into or upon the Demised Premises
(or any part thereof in the name of the whole), and (without being taken or
deemed to be guilty of any manner of trespass or conversion, and without being
liable to indictment, prosecution or damages thereof) may, forcibly if
necessary, expel Tenant and those claiming under Tenant from the Demised
Premises and remove therefrom the effects of Tenant and those claiming under
Tenant.

     19.3 Damages - Termination. Upon the termination of this Lease under the
          ---------------------
provisions of this Article, then except as hereinabove in Section 19.2 otherwise
provided, Tenant shall pay to Landlord the Rent payable by Tenant to Landlord up
to the time of such termination, shall continue to be liable for any preceding
breach of covenant, and in addition, shall pay to Landlord as damages, at the
election of Landlord,

          either:

          (x) the amount by which, at the time of the termination of this Lease
(or at any time thereafter if Landlord shall have initially elected damages
under Subparagraph (y), below), (i) the aggregate of the Rent projected over the
period commencing with such time and ending on the originally scheduled
Termination Date as stated in Article 1 exceeds (ii) the aggregate projected
rental value of the Demised Premises for such period,

                                       29
<PAGE>
 
          or,

          (y) amounts equal to the Rent which would have been payable by Tenant
had this Lease not been so terminated, payable upon the due dates therefor
specified herein following such termination and until the originally scheduled
Termination Date as specified in Article l, provided, however, if Landlord shall
re-let the Demised Premises during such period, that Landlord shall credit
Tenant with the net rents received by Landlord from such re-letting, such net
rents to be determined by first deducting from the gross rents as and when
received by Landlord from such re-letting the expenses incurred or paid by
Landlord terminating this Lease, as well as the expenses of re-letting,
including altering and preparing the Demised Premises for new tenants, brokers'
commissions, and all other similar and dissimilar expenses properly chargeable
against the Demised  Premises and the rental therefrom, it being understood that
any such re-letting may be for a period equal to or shorter or longer than the
remaining term of this Lease; and provided, further, that (i) in no event shall
Tenant be entitled to receive any excess of such net rents over the sums payable
by Tenant to Landlord hereunder and (ii) in no event shall Tenant be entitled in
any suit for the collection of damages pursuant to this Subparagraph (y) to a
credit in respect of any net rents from a re-letting except to the extent that
such net rents are actually received by Landlord prior to such determination.
If the Demised Premises or any part thereof should be re-let in combination with
other space, then proper apportionment on a square foot area basis shall be made
of the rent received from such re-letting and of the expenses of re-letting.

     Suit or suits for the recovery of such damages, or any installments
thereof, may be brought by Landlord from time to time at its election, and
nothing contained herein shall be deemed to require Landlord to postpone suit
until the date when the term of this Lease would have expired if it had not been
terminated hereunder.

     Nothing herein contained shall be construed as limiting or precluding the
recovery by Landlord against Tenant of any sums or damages to which, in addition
to the damages particularly provided above, Landlord may lawfully be entitled by
reason of any default hereunder on the part of Tenant.

     19.4 Fees and Expenses. If Tenant shall default in the performance of any
          -----------------
covenant on Tenant's part to be performed as in this Lease contained, Landlord
may immediately, or at any time thereafter, without notice, perform the same for
the account of Tenant. If Landlord at any time is compelled to pay or elects to
pay any sum of money, or do any act which will require the payment of any sum of
money, by reason of the failure of Tenant to comply with any provision hereof,
or if Landlord is compelled to or does incur any expense, including reasonable
attorneys' fees, in instituting, prosecuting and/or defending any action or
proceeding instituted by reason of any default of Tenant hereunder, Tenant shall
on demand pay to Landlord by way of reimbursement the sum or sums so paid by
Landlord with all interest, costs and damages. Without limiting the generality
of the foregoing, in the event that any Rent is in arrears by more than ten (10)
days after written notice thereof by Landlord to Tenant, Tenant shall pay, as
Additional Rent, a delinquency charge equal to one and one-half percent (1-1/2%)
of the arrearage 

                                       30
<PAGE>
 
for each calendar month (or fraction thereof) during which it remains unpaid.

     19.5 Landlord's Remedies Not Exclusive. The specified remedies to which
          ---------------------------------
Landlord may resort hereunder are cumulative and are not intended to be
exclusive of any remedies or means of redress to which Landlord may at any time
be lawfully entitled, and Landlord may invoke any remedy (including the remedy
of specific performance) allowed at law or in equity as if specific remedies
were not herein provided for.

     19.6 Grace Period. Notwithstanding anything to the contrary in this Article
          ------------
contained, Landlord agrees not to take any action to terminate this Lease (a)
for default by Tenant in the payment when due of Rent, if Tenant shall cure such
default within ten (10) days after written notice thereof given by Landlord to
Tenant, or (b) for default by Tenant in the performance of any other covenant,
if Tenant shall cure such default within a period of thirty (30) days after
written notice thereof given by Landlord to Tenant (except where the nature of
the default is such that remedial action should appropriately take place sooner
as indicated in such written notice), or with respect to covenants other than to
pay a sum of money within such additional period as may reasonably be required
to cure such default if (because of governmental restrictions or any other cause
beyond the reasonable control of Tenant) the default is of such a nature that it
cannot be cured within such thirty (30)-day period, provided, however, (1) that
there shall be no extension of time beyond such thirty (30)-day period for
curing of any such default unless, not more than ten (10) days after the receipt
of the notice of default, Tenant in writing (i) shall specify the cause on
account of which the default cannot be cured during such period and shall advise
Landlord of its intention duly to institute all steps necessary to cure the
default and (ii) shall as soon as may be reasonable, duly institute and
thereafter diligently prosecute to completion all steps necessary to cure such
default and, (2) that no notice of the opportunity to cure default need be
given, and no grace period whatsoever shall be allowed to Tenant, if the default
is incurable or if the covenant or condition, the breach of which gave rise to
the default, had, by reason of a breach on a prior occasion been the subject of
a notice hereunder to cure such default.

20.  END OF TERM - ABANDONED PROPERTY

     Upon the expiration or other termination of the Term of this Lease, Tenant
shall peaceably quit and surrender to Landlord the Demised Premises and all
alterations and additions thereto which Tenant is not entitled or required to
remove under the provisions of this Lease, broom clean in good order, repair and
condition excepting only reasonable use and wear and damage by fire or other
casualty for which, under other provisions of this Lease, Tenant has no
responsibility of repair or restoration.  Tenant's obligation to observe or
perform this covenant shall survive the expiration or other termination of the
term of this Lease.

     Any personal property in which Tenant has an interest which shall remain in
the Building or on the Demised Premises after the expiration or termination of
the Term of this Lease shall be conclusively deemed to have been abandoned, and
may be disposed of in such manner as Landlord may see fit; provided, however,
notwithstanding the foregoing, that Tenant will, upon request of  Landlord made
not later than thirty (30) days after the expiration or termination of the term

                                       31
<PAGE>
 
hereof, promptly remove from the Building any such personal property or, if any
part thereof shall be sold, that Landlord may receive and retain the proceeds of
such sale and apply the same, at its option, against the expenses of the sale,
the cost of moving and storage, any arrears of Rent payable hereunder by Tenant
to Landlord and any damages to which Landlord may be entitled under Article 19
hereof or pursuant to law, with the balance if any, to be paid to Tenant.

21.  RIGHTS OF MORTGAGEES

     21.1 Superiority of Lease. Except as provided in Section 21.7 hereof and to
          --------------------
the extent that it may be provided otherwise by written agreement between Tenant
and a Mortgagee, this Lease shall be superior, and shall not be subordinated, to
a Mortgage or to any other voluntary lien or encumbrance affecting the Land or
Building or any part thereof, provided, however, that such Mortgage shall be
superior, and shall not be subordinated, to this Lease with respect to the
following:

          (a) the prior right and claim under and the prior lien of said
Mortgage in, to and upon any award or other compensation heretofore or hereafter
to be made for any taking by eminent domain of any part of the Demised Premises,
and as to the right of disposition thereof in accordance with the provisions of
the said Mortgage; or

          (b) the prior right and claim under the prior lien of the said
Mortgage, in, to and upon any proceeds payable under all policies of fire and
rent insurance upon the Demised Premises and as to the right of disposition
thereof in accordance with the terms of said Mortgage; and


          (c) any lien, right, power or interest, if any, which may have arisen
or intervened in the period between the recording of the said Mortgage and the
execution of this Lease.

     21.2 Entry and Possession. Upon entry and taking possession of the Property
          --------------------
by a Mortgagee, for the purpose of foreclosure or otherwise, such Mortgagee
shall have all the rights of Landlord, and shall be liable to perform all the
obligations of Landlord arising and accruing during the period of such
possession by such Mortgagee.

     21.3 Right to Cure. No act or failure to act on the part of Landlord which
          -------------
would entitle Tenant under the terms of this Lease, or by law, to be relieved of
Tenant's obligations hereunder or to terminate this Lease, shall result in a
release or termination of such obligations or a termination of this Lease unless
(i) Tenant shall have first given written notice of Landlord's act or failure to
act to first Mortgagees of record, if any, and to any other Mortgagees of record
on the Term Commencement Date of the Lease or whom Tenant has been given written
notice, specifying the act or failure to act on the part of Landlord which could
or would give basis to Tenant's rights; and (ii) such Mortgagees, after receipt
of such notice, have failed or refused to correct or cure the condition
complained of within a reasonable time thereafter, but nothing 

                                       32
<PAGE>
 
contained in this paragraph shall be deemed to impose any obligation on any such
Mortgagees to correct or cure any such condition. "Reasonable time" as used
above means and includes a reasonable time to obtain possession of the Land and
Building if any such Mortgagee elects to do so and a reasonable time to correct
or cure the condition if such condition is determined to exist.

     21.4 Prepaid Rent. No Rent shall be paid more than thirty (30) days prior
          ------------
to the due dates thereof and, as to a first Mortgagee of record and any other
Mortgagees of whom Tenant has been given written notice, payments made in
violation of this provision shall (except to the extent that such rents are
actually received by such Mortgagee) be a nullity as against such Mortgagee and
Tenant shall be liable for the amount of such payments to such Mortgagee.

     21.5 Continuing Offer. The covenants and agreements contained in this Lease
          ----------------
with respect to the rights, powers and benefits of a Mortgagee (particularly,
without limitation thereby, the covenants and agreements contained in this
Article) constitute a continuing offer to any person, corporation or other
entity, which by accepting or requiring an assignment of this Lease or by entry
or foreclosure assumes the obligations herein set forth with respect to such
Mortgagee; every such Mortgagee is hereby constituted a party to this Lease as
an obligee hereunder to the same extent as though its name was written hereon as
such and such Mortgagee shall be entitled to enforce such provisions in its own
name.

     21.6 Subordination. Notwithstanding the foregoing provisions of this
          -------------
Article, Tenant agrees, at the request of Landlord or any Mortgagee, to execute
and deliver promptly any certificate or other instrument which Landlord or such
Mortgagee may request subordinating the Lease and all rights of Tenant under the
Lease to any Mortgage, and to all advances made under such mortgage, provided
that (i) the holder of any such Mortgage shall execute and deliver to Tenant a
nondisturbance agreement to the effect that, in the event of any foreclosure of
such Mortgage, such holder will not name Tenant as a party defendant to such
foreclosure nor disturb its possession under the Lease, or (ii) any such
Mortgage shall contain provisions substantially to the same effect as those
contained in such a nondisturbance agreement.
 
     21.7 Limitations on Liability. Nothing contained in the foregoing Section
          ------------------------
21.6 or in any such nondisturbance agreement or nondisturbance provision shall,
however, affect the prior rights of the holder of any Mortgage with respect to
the proceeds of any award in condemnation or of any fire insurance policies
affecting the Building, or impose upon any such holder any liability (i) for the
erection or completion of the Building, or (ii) in the event of damage or
destruction to the Building or the Demised Premises by fire or other casualty,
for any repairs, replacements, rebuilding or restoration except such repairs,
replacements, rebuilding or restoration as can reasonably be accomplished from
the net proceeds of insurance actually receive by, or made available to, such
holder, or (iii) for any default by Landlord under the Lease occurring prior to
any date upon which such holder shall become Tenant's Landlord, or (iv) for any
credits, offsets or claims against the rent under the Lease as a result of any
acts or omissions of Landlord committed or omitted prior to such date, or (v)
for return of any security deposit or other funds unless the same shall have
been received by such holder, and any such agreement or provision may so state.

                                       33
<PAGE>
 
22.  QUIET ENJOYMENT

     Landlord covenants that if, and so long as, Tenant keeps and performs each
and every covenant, agreement, term, provision and condition herein contained on
the part and on behalf of Tenant to be kept and performed, Tenant shall quietly
enjoy the Demised Premises from and against the claims of all persons claiming
by, through or under Landlord subject, nevertheless, to the covenants,
agreements, terms, provisions and conditions of this Lease and to the mortgages,
ground leases and/or underlying leases to which this Lease is subject and
subordinate.

23.  ENTIRE AGREEMENT - WAIVER - SURRENDER

     23.1 Entire Agreement. This Lease and the Exhibits made a part hereof
          ----------------
contain the entire and only agreement between the parties and any and all
statements and representations, written and oral, including previous
correspondence and agreements between the parties hereto, are merged herein.
Tenant acknowledges that all representations and statements upon which it relied
in executing this Lease are contained herein and that Tenant in no way relied
upon any other statements or representations, written or oral. Any executory
agreement hereafter made shall be ineffective to change, modify, discharge or
effect an abandonment of this Lease in whole or in part unless such executory
agreement is in writing and signed by the party against whom enforcement of the
change, modification, discharge or abandonment is sought. Nothing herein shall
prevent the parties from agreeing to amend this Lease and the Exhibits made part
hereof as long as such amendment shall be in writing and shall be duly signed by
both parties.

     23.2 Waiver by Landlord. The failure of Landlord to seek redress for
          ------------------
violation, or to insist upon the strict performance, of any covenant or
condition of this Lease, or any of the Rules and Regulations promulgated
hereunder, shall not prevent a subsequent act, which would have originally
constituted a violation, from having all the force and effect of an original
violation. The receipt by Landlord of rent with knowledge of the breach of any
covenant of this Lease shall not be deemed a waiver of such breach. The failure
of Landlord to enforce any of such Rules and Regulations against Tenant and/or
any other tenant or subtenant in the Building shall not be deemed a waiver of
any such Rules and Regulations. No provisions of this Lease shall be deemed to
have been waived by Landlord unless such waiver be in writing signed by
Landlord. No payment by Tenant or receipt by Landlord of a lesser amount than
the monthly rent herein stipulated shall be deemed to be other than on account
of the stipulated rent, nor shall any endorsement or statement on any check or
any letter accompanying any check or payment as rent be deemed an accord and
satisfaction, and Landlord may accept such check or payment without prejudice of
Landlord's right to recover the balance of such rent or pursue any other remedy
in this Lease provided.

     23.3 Surrender. No act or thing done by Landlord during the term hereby
          ---------
demised shall be deemed an acceptance of a surrender of the Demised Premises,
and no agreement to accept such surrender shall be valid, unless in writing
signed by Landlord. No employee of Landlord or of Landlord's agents
shall have any power to accept the keys of the Demised Premises prior to the

                                       34
<PAGE>
 
termination of this Lease. The delivery of keys to any employee of Landlord or
of Landlord's agents shall not operate as a termination of the Lease or a
surrender of the Demised Premises. In the event that Tenant at any time desires
to have Landlord underlet the Demised Premises for Tenant's account, Landlord or
Landlord's agents are authorized to receive the keys for such purposes without
releasing Tenant from any of the obligations under this Lease, and Tenant hereby
relieves Landlord of any liability for loss of or damage to any of Tenant's
effects in connection with such underletting.

24.  INABILITY TO PERFORM - EXCULPATORY CLAUSE

     Except as otherwise expressly provided in this Lease, this Lease and the
obligations of Tenant to pay rent hereunder and perform all other covenants,
agreements, terms, provisions and conditions hereunder on the part of Tenant to
be performed shall in no way be affected, impaired or excused because Landlord
is unable to fulfill any of its obligations under this Lease or is unable to
supply or is delayed in supplying any service expressly or impliedly to be
supplied or is unable to make or is delayed in making any repairs, replacements,
additions, alterations, improvements or decorations or is unable to supply or is
delayed in supplying any equipment or fixtures if Landlord is prevented or
delayed from doing so by reason of strikes or labor troubles or any other
similar or dissimilar cause whatsoever beyond Landlord's reasonable control,
including but not limited to, governmental preemption in connection with a
national emergency or by reason of any rule, order or regulation of any
department or subdivision thereof of any governmental agency or by reason of the
conditions of supply and demand which have been or are affected by war,
hostilities or other similar or dissimilar emergency.  In each such instance of
inability of Landlord to perform, Landlord shall exercise reasonable diligence
to eliminate the cause of such inability to perform.

     Tenant shall neither assert nor seek to enforce any claim for breach of
this Lease against any of Landlord's assets other than Landlord's interest in
the Building of which the Premises are a part and in the rents, issues and
profits thereof, and Tenant agrees to look solely to such interest for the
satisfaction of any liability of Landlord under this Lease, it being
specifically agreed that in no event shall Landlord (which term shall include
without limitation any of the officers, trustees, directors, partners,
beneficiaries, joint venturers, members, stockholders or other principals or
representatives, disclosed or undisclosed of Landlord or any managing agent)
ever be personally liable for any such liability.  This paragraph shall not
limit any right that Tenant might otherwise have to obtain injunctive relief
against Landlord or to take any other action which shall not involve the
personal liability of Landlord to respond in monetary damages from Landlord's
assets other than the Landlord's interest in said real estate, as aforesaid.  In
no event shall Landlord ever be liable for consequential damages.

25.  BILLS AND NOTICES

     Any notice, consent, request, bill, demand or statement hereunder by either
party to the other party shall be in writing and, if received at Landlord's or
Tenant's address, shall be deemed to have been duly given when either delivered
or served personally or mailed in a postpaid envelope, deposited in the United
States mails addressed to the respective party at its address as 

                                       35
<PAGE>
 
stated in Article 1, or if any address for notices shall have been duly changed
as hereinafter provided, if mailed as aforesaid to the party at such changed
address. Either party may at any time change the address for such notices,
consents, requests, bills, demands or statements by delivering or mailing, as
aforesaid, to the other party a notice stating the change and setting forth the
changed address, provided such changed address is within the United States.

     All bills and statements for reimbursement or other payments or charges due
from Tenant to Landlord hereunder shall set forth in reasonable detail the
particulars relating thereto and shall be due and payable in full thirty (30)
days, unless herein otherwise provided, after submission thereof by Landlord to
Tenant.  Landlord shall, at Tenant's reasonable request, promptly furnish Tenant
with such additional reasonable details relating thereto as Tenant shall
reasonably request.  Tenant's failure to make timely payment of any amounts
indicated by such bills and statements, whether for work done by Landlord at
Tenant's request, reimbursement provided for by this Lease or for any other sums
properly owing by Tenant to Landlord, shall be treated as a default in the
payment of Rent, in which event Landlord shall have all rights and remedies
provided in this Lease for the nonpayment of Rent.  Unless otherwise directed by
Tenant, a copy of all default notices to Tenant shall be forwarded to Director
of Real Estate, Focal Communications Corporation, 200 North LaSalle Street,
Suite 800, Chicago, Illinois 60601.

26.  PARTIES BOUND - SEIZING OF TITLE

     The covenants, agreements, terms, provisions and conditions of this Lease
shall bind and benefit the successors and assigns of the parties hereto with the
same effect as if mentioned in each instance where a party hereto is named or
referred to, except that no violation of the provisions of Article 14 hereof
shall operate to vest any rights in any successor or assignee of Tenant and that
the provisions of this Article shall not be construed as modifying the
conditions of limitation contained in Article 19 hereof.

     If in connection with or as a consequence of the sale, transfer or other
disposition of the real estate (Land and/or Building either or both, as the case
may be) of which the Demised Premises are a part Landlord ceases to be the owner
of the reversionary interest in the Premises, Landlord shall so notify Tenant
and Landlord shall be entirely freed and relieved from the performance and
observance thereafter of all covenants and obligations hereunder accruing
thereafter on the part of Landlord to be performed and observed, it being
understood and agreed in such event (and it shall be deemed and construed as a
covenant running with the land) that the person succeeding to Landlord's
ownership of said reversionary interest shall thereupon and thereafter assume,
and perform and observe, any and all of such covenants and obligations of
Landlord.

27.  MISCELLANEOUS

     27.1 Separability. If any provision of this Lease or portion of such
          ------------
provision or the application thereof to any person or circumstance is for any
reason held invalid or unenforceable, the remainder of the Lease (or the
remainder of such provision) and the application thereof to 

                                       36
<PAGE>
 
other persons or circumstances shall not be affected thereby.

     27.2 Captions.  The captions are inserted only as a matter of convenience
          --------                                                              
and for reference, and in no way define, limit or describe the scope of this
Lease nor the intent of any provision thereof.

     27.3 Broker.  Each party represents and warrants that it has not directly
          ------                                                                
or indirectly dealt, with respect to the leasing of office space in the
Building, with any broker or had its attention called to the Premises or other
space to let in the Building, by any broker other than the Broker(s) (if any)
listed in Article 1 whose commission shall be the responsibility of Landlord.
Each party agrees to exonerate and save harmless and indemnify the other against
any claims for a commission by any other broker, person or firm with whom such
party has dealt in connection with the execution and delivery of this Lease or
out of negotiations between Landlord and Tenant with respect to the leasing of
other space in the Building.

     27.4 Governing Law. This lease is made pursuant to, and shall be governed
          -------------
by, and construed in accordance with, the laws of the Commonwealth of
Massachusetts.

     27.5 Assignment of Rents. With reference to any assignment by Landlord of
          -------------------
its interest in this Lease, or the Rent payable hereunder, conditional in nature
or otherwise, which assignment is made to or held by a bank, trust company,
insurance company or other institutional lender holding a Mortgage on the
Building, Landlord and Tenant agree:

          (a) that the execution thereof by Landlord and acceptance thereof by
such Mortgagee shall never be deemed an assumption by such Mortgagee of any of
the obligations of the Landlord thereunder, unless such Mortgagee shall, by
written notice sent to the Tenant, specifically otherwise elect; and

          (b) that, except as aforesaid, such Mortgagee shall be treated as
having assumed the Landlord's obligations thereunder only upon foreclosure of
such Mortgagee's Mortgage and the taking of possession of the Demised Premises
after having given notice of its exercise of the option stated in Article 21
hereof to succeed to the interest of the Landlord under this Lease.

     27.6 Parking.  Landlord shall allocate to Tenant the number of non-
          -------
exclusive Parking Spaces indicated in Article 1.  Landlord may, pursuant to
Section 15.1, establish Rules and Regulations relative to all parking areas
serving Building tenants and may further engage the services of an independent
contractor to administer and control access to said parking areas.  Landlord or
said independent contractor shall impose separate charges for use of said
parking areas, and such charges shall be payable by Tenant as Additional Rent
with respect to Tenant's Parking Spaces.  As of the Term Commencement Date the
monthly charge for each Parking Space situated in the garage facility serving
the Building shall be One Hundred Sixty Dollars ($160).  Thereafter, the monthly
charge shall be as from time to time established by Landlord or said independent
contractor as the then prevailing monthly parking charge for the Building.
Tenant acknowledges that Landlord has informed Tenant that Landlord intends to
allocate in its 

                                       37
<PAGE>
 
tenant leases up to one hundred twenty-five percent (125%) of the actual parking
spaces servicing the Building. It is further acknowledged and agreed that as a
consequence of such over-allocation of parking spaces there may occasionally
occur instances in which the number of parking spaces actually available to
Tenant shall be less than the Parking Spaces to which Tenant is entitled under
this Lease. Landlord shall incur no liability to Tenant as a consequence of such
over-allocation of parking spaces.

     27.7 Notice of Lease. Neither party shall record this Lease in any Registry
          ---------------
of Deeds or Registry District; provided, however, that concurrently with the
execution hereof, Landlord and Tenant shall execute and deliver a recordable
Notice of Lease in the form prescribed by Chapter 183, Section 4 of the
Massachusetts General Laws for filing with Middlesex South Registry of Deeds.

     27.8 Financial Statements. If requested by Landlord, Tenant shall furnish
          --------------------
to Landlord promptly after they are available to Tenant copies of Tenant's
annual financial statements (audited, if available) and unaudited monthly
financial statements and such other financial statements as Tenant shall furnish
from time to time to any lender and/or equity holder of Tenant. It is understood
and agreed that Landlord may furnish copies of any and all of such financial
statements to one or more of its mortgagees.

     27.9 Holding Over. Any holding over by Tenant after the expiration of the
          ------------
Term of this Lease shall be treated as a tenancy at sufferance at twice the
Yearly Fixed Rent and Additional Rent herein provided (prorated on a daily
basis) and shall otherwise be on the terms and conditions set forth in this
Lease, as far as applicable.

                                       38
<PAGE>
 
     IN WITNESS WHEREOF, Landlord and Tenant have caused this instrument to be
executed under seal, all as of the day and year first above written.


                         RIVERFRONT OFFICE PARK ASSOCIATES II
                         LIMITED PARTNERSHIP

                         By:  DARVEL REALTY TRUST
                               Managing General Partner


                         By: /s/ Michael P. Sullivan
                            ------------------------------------------------
                            Michael P. Sullivan, Vice President


                         FOCAL COMMUNICATIONS CORPORATION

 
                         By: /s/ Brian F. Addy
                            ------------------------------------------------
                            Its Executive Vice President

                                       39
<PAGE>
 
                                   EXHIBIT A
                                   ---------

                       PLAN OF FOCAL COMMUNICATION CORP.
                                   13,274 SF
<PAGE>
 
                                   EXHIBIT B
                                   ---------

                              DESCRIPTION OF LAND
                              -------------------


          A certain parcel of land in the City of Cambridge, Middlesex County,
Massachusetts, bounded and described as follows:

     SOUTHWESTERLY            at the intersection of Main Street and First
                              Street, by four lines measuring 44.72 feet, 129.49
                              feet, 24.15 feet, and 41.16 feet, respectively;

     SOUTHERLY                by Main Street by a line measuring 404.86 feet;

     WESTERLY                 by Lot 1, by a line measuring 154.28 feet; and

     NORTHERLY                by the Broad Canal, 594.60 feet.

     Said parcel is shown as Lot 2 on a "Subdivision Plan of Land in Cambridge,
Mass. (Middlesex County)", dated April 24, 1981, and revised September 4, 1981,
drawn by Boston Survey Consultants, and prepared for Darvel Realty Trust,
recorded with Middlesex Southern District Registry of Deeds in Book 14412, Page
199.

<PAGE>
 
                                   EXHIBIT C
                                   ---------

                               AUDIT PROCEDURES
                               ----------------


1.   Such documentation and calculation shall be made available to Tenant at the
offices where Landlord keeps such records in Cambridge/Boston, Massachusetts
during normal business hours within a reasonable time (not to exceed ten (10)
business days) after Landlord receives a written request from Tenant to make
such examination.

2.   Tenant shall have the right to make such examination no more than once in
respect of any fiscal tax year or calendar operation year, as the case may be,
in which Landlord has given Tenant a statement of the actual amount of taxes or
operating expenses, as the case may be, for such year.

3.   Any request for examination in respect of any such calendar or fiscal year
shall be made no more than sixty (60) days after Landlord advises Tenant of the
actual amount of taxes or operating expenses in respect of such period.

4.   Such examination may be made only by an independent certified public
accounting firm approved by Landlord, which consent shall not be unreasonably
withheld or delayed, paid by Tenant on a straight fee (non-contingent fee)
basis.

5.   As a condition of performing any such examination, Tenant and its examiners
shall be required to execute and deliver to Landlord an agreement, in form
reasonable acceptable to Landlord, agreeing to keep confidential any information
which it discovers about Landlord or the Building in connection with such
examination.

<PAGE>
 
                                   EXHIBIT D
                                   ---------


                     PROCEDURE FOR ALLOCATION OF COSTS OF
                        ELECTRIC POWER USAGE BY TENANTS

     1.  Main electric service to the Building is provided and metered by the
local utility company.  Building energy consumption via switch gear MS1 & MS1A
will be billed to and paid for by Landlord at rates established by the utility
company.

     Tenant shall pay for its electricity directly to the electric utility
supplying power for dedicated preferred source switch gear MS2 (room 126) fed
from Cambridge Electric Light Company (CELCO circuit #55).  This power source is
dedicated solely for operation of the computer facilities on the eleventh (11th)
floor of the Building and the associated chilled water plant located on the
fourteenth (14th) floor and the roof of the Building and is separately metered
by Cambridge Electric Light Company (meter #8016527).

     (Separate electric service from the utility company directly to tenants may
be made available to retail tenants on the ground floor if Landlord determines
this to be appropriate to the tenancy involved.)

     2.  In order to allocate charges for electric service fairly among tenants
in relation to the relative amounts of electricity used by each tenant,
additional meters (known as "check meters") have been installed by Landlord for
each tenant to measure all electricity provided for lights and power to that
tenant.  This shall not, however, include the following, which shall be wired
from the main building service and not through the check meters:  stairwell and
emergency lights; elevators; heat pumps and HVAC in the Building; exterior
lighting; and all main building mechanical systems (common areas on each floor,
including the elevator lobby, corridors, and bathrooms, will have service
through the check meters on each floor) (the "Basic Building Electricity") and
which shall be separately metered.

     3.  Additional check meters may be installed by Landlord where necessary to
assure measurement of all electric service to tenant areas (e.g., in the case of
separate dedicated circuits to computer rooms, cafeterias, or other special
purpose facilities).  Ground floor tenant space will be check metered if it is
not separately metered.  In addition, further modification to the number and
location of check meters may be made by Landlord if required to improve the
quality of information obtained thereby.

     4.  Landlord will cause the check meters to be read monthly by its
employees and will perform an analysis of the information for the purpose of
determining an equitable allocation of the costs of electric service among the
tenants in the Building in relation to the respective amounts of usage of
electricity by those tenants.

     5.  Each tenant's allocable share ("Tenant's Allocable Electricity Cost"),
shall be determined by Landlord on the following basis:
<PAGE>
 
     a.        The total kilowatt hour usage for the period under evaluation
               shall be established for each check meter and also for the
               Building as a whole by a reading of the main building meter for
               that period.

     b.        The cost of the total amount of electricity supplied for usage by
               tenants during the period (exclusive of the Base Building
               Electricity) (herein called "Tenant Electricity") shall be
               determined by multiplying the total cost of electricity as
               invoiced by the utility company for the same period by a
               fraction, the numerator of which is the total amount of kilowatt
               hour usage as measured by all of the Tenant Electricity check
               meters in the Building and the denominator of which is the total
               amount of kilowatt hour usage for the Building as measured by the
               main building electric meter.

     c.        Tenant's Allocable Electricity Cost for the period shall be
               determined by multiplying the total costs of Tenant Electricity
               by a fraction, the numerator of which is the kilowatt hour usage
               of Tenant Electricity by said tenant (calculated as the sum of
               kilowatt hour usage during the period measured by all check
               meters serving its premises) and the denominator of which is the
               total kilowatt hour usage of Tenant Electricity for the same
               period.

     d.        Where part or all of the rentable area on a floor has been
               occupied by a tenant for less than all of the period for which
               said Tenant's Allocable Electricity Cost is being calculated,
               appropriate and equitable modifications shall be made to the
               allocation formula so that each tenant's allocable share of costs
               equitably reflects its period of occupancy, provided that in no
               event shall the total of all costs as allocated to tenants be
               less than the total cost of Tenant Electricity for said period.

     6.  All costs of Base Building Electricity to Landlord shall be treated as
part of the Operating Costs of the Building for purposes of determining the
allocation of those costs.

                                       2
<PAGE>
 
                                   EXHIBIT E
                                   ---------

                            CLEANING SPECIFICATIONS
                            -----------------------

OFFICE AREA:
----------- 

DAILY - Monday through Friday inclusive, Massachusetts legal holidays excluded.
-----                                                                          

1.   Empty all waste receptacles and remove trash to designated area, wash
     receptacles as necessary.

2.   Empty and wipe all ashtrays.

3.   Vacuum all rugs and carpets.

4.   Hand dust and wipe clean with treated cloths all horizontal surfaces,
     including furniture, office equipment, window sills, door ledges, chair
     rails and converter tops, within normal reach.

5.   Wash and clean all bathrooms and water fountains.

6.   Remove and dust under all desk equipment and telephones and replace same.

7.   Wipe clean all brass and other bright work.

8.   Sweep and dust mop all uncarpeted areas using a dust treated mop.

9.   Hand dust all grill work within normal reach.

WEEKLY
------

1.   Dust coat racks and the like.

2.   Remove all finger marks from private entrance doors, light switches and
     doorways.

QUARTERLY - Render high dusting not reached in daily cleaning to include:
---------                                                                

1.   Dusting all pictures frames, charts, graphs and similar wall hangings.

2.   Dusting all vertical surfaces such as walls, partitions and doors.

3.   Dusting of all venetian blinds.

TWICE PER YEAR - Wash interior and exterior of perimeter windows.
--------------                                                   
<PAGE>
 
MAIN LOBBY, ELEVATORS, BUILDING EXTERIOR AND CORRIDORS - CONTINUED
------------------------------------------------------------------

4.   Spot clean any metal work in lobby.

5.   Spot clean any metal work surrounding building entrance doors.

6.   Spot clean and polish guard desk.

7.   Vacuum all corridor carpets.

MONTHLY
-------

1.   All resilient tile floors in public area are to be treated equivalent to
     spray buffing.

2.   Shampoo carpeting in elevator.

STAIRWELLS
----------

DAILY - Monday through Friday inclusive, Massachusetts legal holidays excluded.
-----                                                                          

1.   Sweep all stairwells.


                                       2
<PAGE>
 
                                   EXHIBIT F
                                   ---------

                             RULES AND REGULATIONS
                             ---------------------


     1.   The sidewalks, entrances, passages, courts, elevators, vestibules,
stairways, corridors or halls of the Building shall not be obstructed or
encumbered or used for any purpose other than ingress and egress to and from the
premises demised to any tenant or occupant.

     2.   No awnings or other projections shall be attached to the outside walls
or windows of the Building without the prior consent of Landlord.  No curtains,
blinds, shades, or screens shall be attached or hung in, or used in connection
with, any window or door of the premises demised to any tenant or occupant,
without the prior consent of Landlord.  Such awnings, projections, curtains,
blinds, shades, screens, or other fixtures must be of a quality type, design and
color, and attached in a manner, approved by Landlord.

     3.  No sign, advertisement, object, notice or other lettering shall be
exhibited, inscribed, painted or affixed on any part of the outside or inside of
the premises demised to any tenant or occupant or of the Building without the
prior written consent of Landlord.  Interior signs on doors and directory
tables, if any, shall be of a size, color and style approved by Landlord.

     4.  The sashes, sash doors, skylights, windows, and doors that reflect or
admit light and air into the halls, passageways or other public places in the
Building shall not be covered or obstructed, nor shall any bottles, parcels, or
other articles be placed or stored upon on any window sills.

     5.  No show cases or other articles of any kind shall be put in front of or
affixed to any part of the exterior of the Building, nor placed in the halls,
corridors, vestibules or other parts of the Building.

     6.  The water and wash closets and other plumbing fixtures shall not be
used for any purposes other than those for which they were constructed, and no
sweepings, rubbish, rags, or other substances shall be thrown therein.

     7.  No tenant or occupant shall mark, paint, drill into, or in any way
deface any part of the Building or the premises demised to such tenant or
occupant.  No boring, cutting or stringing of wires shall be permitted, except
with the prior consent of the Landlord, and as Landlord may direct.  No tenant
or occupant shall install any resilient tile or similar floor covering in the
premises demised to such tenant or occupant except in manner approved by
Landlord.

     8.  No bicycles, vehicles or animals of any kind shall be brought into or
kept in or about the premises demised to any tenant. Bicycles may be stored in
racks, if any, furnished for such purpose by Landlord in a common area of the
Building. No cooking shall be done or permitted in the Building by any tenant
without the approval of Landlord. No tenant shall cause or permit any unusual or
objectionable odors to emanate from the premises demised to such
<PAGE>
 
tenant.

     9.   Without the prior consent of Landlord, no space in the Building shall
be used for manufacturing, or for the sale of merchandise, goods or property of
any kind at auction.

     10.  No tenant shall make, or permit to be made, any unseemly or disturbing
noises or disturb or interfere with other tenants or occupants of the Building
or neighboring buildings or premises, whether by the use of any musical
instrument, radio, television set or other audio device, unmusical noise,
whistling, singing, or in any other way.  Nothing shall be thrown out of any
doors or windows.

     11.  Each tenant must, upon the termination of its tenancy, restore to
Landlord all keys, either furnished to, or otherwise procured by, such tenant,
including without limitation, all parking pass keys, Building keys, office keys
and keys to storage areas and toilet rooms.

     12.  All removals from the Building, or the carrying in or out of the
Building or the premises demised to any tenant, of any safes, freight,
furniture, or bulky matter of any description must take place at such time and
in such manner as Landlord or its agents may determine, from time to time.
Landlord reserves the right to inspect all freight to be brought into the
Building and to exclude from the Building all freight which violates any of the
Building Rules or the provisions of such tenant's lease.

     13.  No tenant shall use or occupy, or permit any portion of the premises
demised to such tenant to be used or occupied, as an office for a public
stenographer or typist, or to a barber or manicure shop, or as an employment
bureau.  No tenant or occupant shall engage or pay any employees in the
Building, except those actually working for such tenant or occupant in the
Building, nor advertise for laborers giving an address at the Building.

     14.  No tenant or occupant shall purchase spring water, ice, food,
beverage, lighting maintenance, cleaning towels or other like service, from any
company or person not approved by Landlord, such approval not unreasonably to be
withheld.

     15.  Landlord shall have the right to prohibit any advertising by any
tenant or occupant which, in Landlord's opinion, tends to impair the reputation
of the Building or its desirability as a building for offices, and upon notice
from Landlord, such tenant or occupant shall refrain from or discontinue such
advertising.

     16.  Landlord reserves the right to exclude from the Building, between the
hours of 6:00 p.m. and 8:00 a.m. on Business Days and otherwise at all hours,
all persons who do not present a pass to the building signed by the Landlord.
Landlord will furnish passes to persons for whom any tenant requests such
passes.  Each tenant shall be responsible for all persons for whom it requests
such passes and shall be liable to Landlord for all wrongful acts of such
persons.

                                       3
<PAGE>
 
     17.  Each tenant, before closing and leaving the premises demised to such
tenant at any time, shall see that all entrance doors are locked and windows
closed.

     18.  Each tenant shall, at its expense, provide artificial light in the
premises demised to such tenant for Landlord's agents, contractors, and
employees while performing janitorial or other cleaning services and making
repairs or alterations in said premises.

     19.  No premises shall be used, or permitted to be used, for lodging or
sleeping, or for any immoral or illegal purpose.

     20.  There shall not be used in the Building, either by any tenant or
occupant or by their agents or contractors, in the delivery or receipt of
merchandise, freight or other matter, any hand trucks or other means of
conveyance, except those equipped with rubber tires, rubber side guards and such
other safeguards as Landlord may require.

     21.  Canvassing, soliciting and peddling in the Building are prohibited and
each tenant and occupant shall cooperate in seeking their prevention.

     22.  If the premises demised to any tenant become infested with vermin,
such tenant, at its sole cost and expense, shall cause its premises to be
exterminated from time to time, to the satisfaction of Landlord and shall employ
such exterminators therefor as shall be approved by Landlord.

     23.  No premises shall be used, or permitted to be used, at any time,
without the prior approval of Landlord, as a store for the sale or display of
goods, wares or merchandise of any kind, or as a restaurant, shop, booth,
bootblack or other stand, or for the conduct of any business or occupation which
predominantly involves direct patronage of the general public in the premises
demised to such tenant, or for manufacturing or for other similar purposes.

     24.  No tenant shall move, or permit to be moved, into or out of the
Building or the premises demised to such tenant, any heavy or bulky matter,
without the specific prior written approval of Landlord.  If any such matter
requires special handing, only a person holding a Master Rigger's License shall
be employed to perform such special handling.  No tenant shall place, or permit
to be placed on any part of the floor or floors of the premises demised to such
tenant, a load exceeding the floor load per square foot which such floor was
designed to carry and which is allowed by law.  Landlord reserves the right to
prescribe the weight and position of safes and other heavy matter, which must be
placed so as to distribute the weight.

     25.  The requirements of tenants will be attended to only upon application
at the office of the Building.  Building employees shall not be required to
perform, and shall not be requested by any tenant or occupant to perform any
work outside of their regular duties, unless under specific instructions from
the office of the managing agent of the Building.

                                       4