Exhibit 10a





                               INDENTURE OF LEASE

                                 BY AND BETWEEN

                            RIVERTECH ASSOCIATES LLC

                                   ("LESSOR")

                                       AND

                                  DYNAGEN, INC.

                                   ("LESSEE")


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                           RIVERSIDE TECHNOLOGY CENTER

                               840 MEMORIAL DRIVE
                            CAMBRIDGE, MASSACHUSETTS


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Lease Dated July 1, 1997









                           RIVERSIDE TECHNOLOGY CENTER
                                COMMERCIAL LEASE
                                     BETWEEN
                            RIVERTECH ASSOCIATES, LLC
                                       AND
                                  DYNAGEN, INC.

         Agreement  entered into this 1st day of July, 1997 in  consideration of
the covenants and other benefits herein  contained,  the receipt and sufficiency
of said consideration being hereby acknowledged.

         Rivertech  Associates LLC, a Massachusetts  limited liability  company,
c/o The Abbey Group 575 Boylston Street Boston, MA 02116 (herein "LESSOR"), does
hereby lease to DynaGen, Inc., a Delaware corporation having its principal place
of business at 99 Erie Street Cambridge, Massachusetts 02139, (herein "Lessee"),
and LESSEE does hereby  lease from said  LESSOR,  certain  space  located at 840
Memorial Drive, Cambridge, Massachusetts (herein "Building"), being that portion
of the forth floor of the Building shown on Exhibit A attached  hereto  (herein,
"Lease  Plan')  consisting  of  approximately  12,262  rentable  square  feet as
appearing on said Lease Plan, (the "Leased or Premises" or "Premises"); with the
right in common with others in the Building to use (a) the parking  areas on the
parcel of land on which  the  Building  is  located  or  otherwise  serving  the
Building (as said parking  areas are  expressly  contemplated  herein);  (b) the
driveways  and  walkways  necessary  for access to the  Building or such parking
areas; (c ) the entrances,  lobbies,  stairs,  passenger elevators and corridors
necessary  for  access  to the  Premises;  (d) the  loading  docks  and  freight
elevators in the Building or such parking areas;  (c) Premises;  (d) the loading
docks and freight  elevators  in the  Building,  subject to LESSOR's  reasonable
rules and regulations promulgated according to section 24 of this Lease; (e) the
lavatories  on the fourth floor of the Building;  (f) the heating,  ventilation,
air conditioning,  plumbing, electrical,  emergency and other mechanical systems
and equipment serving the Premises in common with other portions of the Building
(as more  specifically  set forth  herein);  and (g) such other common areas and
facilities as LESSOR may designate from time to time (collectively,  the "Common
Areas").

         1. TERM. LESSEE leases the Leased Premises for an original Term of five
(5) years  (herein,  "Lease  Term").  The Term of the Lease  shall  begin on the
Commencement Date (as hereinafter defined), and shall end on the last day of the
calendar  month  which  is  sixty  (60)  full  calendar  months  therefrom  (the
"Termination Date").

         The first full twelve (12) month period following the Commencement Date
(or,  if the  Commencement  Date  occurs on a date other than the first day of a
calendar month, the first day of the first calendar month after the Commencement
Date occurs) and each subsequent twelve month period during the Lease Term shall
be called "Lease Years".  To 










                                      -2-


the extent the Commencement  Date occurs on any date other than the first of any
month,  the first Lease Year shall be enlarged to include the period between the
Commencement Date and the beginning of the first full month of the Lease Term.

         The Commencement Date shall be that date upon which the LESSOR delivers
the Office Space (as  hereinafter  defined) to LESSEE,  as evidenced by LESSOR's
tender of its Phase I Delivery Notice as contemplated by paragraph 33 hereof, in
broom clean condition,  and free of all tenants and occupants,  and with Phase I
substantially complete (as those terms are hereinafter defined).

         For example,  if the Commencement Date is September 15, 1997, the first
Lease year begins on October 1, 1997 and ends  September  30,  1998;  the second
Lease Year begins on October 1, 1998 and ends  September 30, 1999;  and so on up
to the Termination Date of September 30, 2002.

         2. RENT.  Beginning as of the  Commencement  Date,  LESSEE shall pay to
LESSOR rent at an annual rate  pursuant to the schedule  below during each Lease
Year (or portion thereof as the case may be) of the Lease Term hereof,  (herein,
"Annual Base  Rent").  Annual Base Rent shall be payable,  in advance,  in equal
monthly  installments,  due on the first day of each calendar month, pursuant to
the schedule below.

         Notwithstanding any other provision of this Lease,  because Annual Base
Rent is to be paid on the basis of a "Lease Year" which is defined  above as the
first  consecutive  twelve (12) months after the Commencement  Date, and because
the  Commencement  Date may be a date  other  than the first  day of a  calendar
month, there shall be an incremental  installment (or installments) of rent (the
"Interim Rent'),  which is based on Annual Base Rent for the first Lease Year as
set forth in the schedule  below and  reflective  of the delivery of the Demised
Premises in two phases; determined as follows:

         The monthly  installment  of Interim Rent shall consist of Phase I Rent
and Phase II Rent. Rent due for each applicable month from the Commencement Date
(i.e.  the "Phase I Delivery  Date")  until the Phase II Delivery  Date shall be
$26,142.53 (the "Phase I Rent"); and accordingly, that monthly rate equals a per
diem rate of $871.42. The "Phase II Delivery Date" shall be that date upon which
the LESSOR  delivers  the balance of the Leased  Premises  other than the Office
Space,  to LESSEE,  as  evidenced  by  LESSOR's  tender of its Phase II Delivery
Notice as contemplated  by paragraph 33 hereof,  in broom clean  condition,  and
free of all tenants and occupants,  and with Phase II substantially complete (as
those terms are  hereinafter  defined).  LESSEE shall pay to LESSOR Phase I Rent
based on the above stated per diem rate,  for each day between the  Commencement
Date and the Phase II Delivery Date.  (Payment shall be made in advance,  by the
first of each calendar  month, in the full amount of the Phase I Rent, (for each
month  starting  September 1, 1997 where the Phase II Delivery  Date has not yet
occurred)  with there to be an  appropriate  adjustment  made by any portions of
that month where Interim Rent  consists of Phase I Rent and Phase II Rent,  said
adjustment to be added to or  subtracted  from the first  installment  of Annual
Base Rent due the next full calendar month, 










                                      -3-

and promptly credited or paid as the case may be). Additionally, for the balance
of any calendar  month in which the Phase II Delivery  Date occurs,  through the
end of said calendar month,  LESSEE's  payment of Interim Rent shall be based on
the full  Annual Base Rent for the first  Lease Year  pursuant  to the  schedule
below, in a monthly amount of $34,231.42 (the "Phase II Rent"), and accordingly,
that  monthly  rate  equals a per diem rate of  $1,141.05.  LESSEE  shall pay to
LESSOR  Phase II Rent  based on the above  stated  per diem  rate,  for each day
between the Phase II Delivery  Date through the last day of the calendar  month.
The next payment then due shall be the first monthly  installment of Annual Base
Rent on or before the first of the calendar month.

         Any and all charges for  Additional  Operating  Expense  Rent and other
Additional Rent as hereinafter defined and due after the Commencement Date shall
also be prorated on a daily basis reflective of the space delivered.

         All payments of Interim Rent (i.e.  Phase I Rent and Phase II Rent) and
Annual Base Rent; and also  Additional  Operating  Expense Rent,  Additional Tax
Escalation Rent, and any and all other sums and may be due from LESSEE to LESSOR
under this Lease, all of which shall be deemed "Additional Rent'; (the foregoing
to be  singularly as to any item or  collectively  referred to herein as "Rent")
shall be made to LESSOR's agent,  the Abbey  Corporation,  575 Boylston  Street,
Boston,  Massachusetts  02116 or to such other  agent or at such other  place as
LESSOR may designate in writing.

         LESSEE  shall  pay  interest  from the date due,  at an annual  rate of
fifteen  (15%) percent for any  installments  of Annual Base Rent, or Additional
Rent or other  payments  due from  LESSEE to LESSOR  which are not  received  by
LESSOR within ten days after  written  notice from LESSOR that Annual Base Rent,
or Additional Rent or other payments were not received

                       SCHEDULE OF ANNUAL BASE RENT

     Lease Year               Annual Base Rent           Monthly Installment

    Lease Year 1                 $410,777.00                $34,231.42  
    Lease Year 2                 $416,908.00                $34,742.33
    Lease Year 3                 $423,039.00                $35,253.25
    Lease Year 4                 $429,170.00                $35,764.17
    Lease Year 5                 $435,301.00                $36,275.08


         3.  ADDITIONAL  RENT.  (Operating  Expenses).   Commencing  as  of  the
Commencement  Date,  LESSEE, in addition to the sums payable to LESSOR as Annual
Base  Rent as  determined  in  Section 2  hereof,  shall pay to LESSOR  for each
calendar  year (or  portion  thereof,  as  applicable)  of the  Lease  Term,  as
additional rent, LESSEE's Allocable  percentage (as hereinafter  defined) of any
and all increases in operating  expenses 









                                      -4-

attributable  to  the  Building  for  said  year  of  the  Lease  Term  (herein,
"Additional Operating Expense Rent').

         The base from which the amount of any  increases in operating  expenses
shall be determined is the operating expense schedule compiled and calculated by
LESSOR,  representing  actual operating  expenses for calendar year 1996, as set
forth as  Exhibit B  attached  hereto  and  incorporated  herein,  which  LESSOR
represents a complete and correct  summary of said  operating  expense  amounts.
LESSEE's  payment  shall be prorated  if the Lease  commences  or is  terminated
(without breach) during any fiscal year.

         Solely for the purpose of calculating  LESSEE's Allocable Percentage of
increases in operating expenses over those shown in Exhibit B hereto,  operating
expense  increases  shall not  include  the  following:  The  costs of  LESSEE's
improvements  performed by LESSOR (if any), or as they may be approved by LESSOR
in  the  future;  all  items  and  services  for  which  LESSEE  or  any  tenant
specifically and directly  reimburses  LESSOR, or pays third persons at LESSOR's
directions;  income or franchise taxes of the LESSOR;  the costs incurred in any
rehabilitation,   reconstruction,  or  other  work  occasioned  by  any  insured
casualty,  or by the  exercise  of the right of  eminent  domain,  except to the
extent of any so-called  "deductible" amount under policies of insurance (not to
exceed  $25,000.00)  or any costs  actually  incurred  for  which any  insurance
company does not  reimburse or  compensate  LESSOR  (unless  LESSOR was required
under this Lease to maintain  insurance  covering  such costs);  and those other
items  listed in  Exhibit  B-1  hereto  Notwithstanding  the  foregoing,  LESSOR
reserves the right to claim as against LESSEE as provided  under this Lease,  or
any other  tenant or  individual  or entity  for any and all costs and  expenses
caused directly or occasioned by their willful  misconduct or other negligent or
wrongful acts or omissions.

         Operating expense escalation is ordinarily invoiced to LESSEE by LESSOR
in January of each calendar year. LESSEE shall not be responsible for payment of
any increase in operating  expense  escalations  for calendar  year 1997 greater
than five (5%) percent over said  operating  expenses for calendar year 1996. If
the Lease Term includes any partial  calendar  year,  the  Additional  Operating
Expense Rent for such calendar year shall be prorated  according to the fraction
of the total days in such  calendar  year that are  included  in the Lease Term.
LESSOR shall have the option to require LESSEE to pay the  Additional  Operating
Expense Rent in any Lease year in equal monthly installments on the first day of
each calendar  month during the Lease Term;  provided  LESSOR first  delivers to
LESSEE its  statement  of the monthly  amount due for the LEASE Year in question
based on LESSOR's reasonable estimates. Within ninety (90) days after the end of
each  calendar  year  included  within the Lease Term,  LESSOR shall  deliver to
LESSEE a written  statement  of the  actual  operating  expenses  and the actual
Additional  Operating  Expense Rent for such year. If such  statement  indicates
that the  actual  Additional  Operating  Expense  Rent for  such  calendar  year
exceeded LESSEE's estimated payments on account of Additional  Operating Expense
Rent for such calendar year,  then LESSEE shall pay such excess to LESSOR within
thirty (30) days of its receipt of such statement from LESSOR. If such statement
indicates that LESSEE's  estimated  payments on account of









                                      -5-

Additional  Operating  Expense Rent for such  calendar  year exceeded the actual
Additional  Operating  Expense Rent for such calendar year,  LESSOR shall credit
the excess against the next installment(s) of Additional  Operating Expense Rent
due from LESSEE,  or at the end of the Lease Term pay such excess to LESSEE when
delivering such statement to LESSEE.

         LESSEE's  Allocable  Percentage for the purpose of this Lease is 9.73%;
provided  however,  for periods up to the Phase II Delivery  Date, the Allocable
Percentage to be applied shall be 7.75%.

         4. ADDITIONAL RENT (Tax Escalation).  Commencing as of the Commencement
Date,  LESSEE,  in addition to the sums payable to LESSOR as Annual Base Rent as
determined in Section 2 hereof, and in addition to the sums payable to LESSOR as
determined  in Section 3 hereof,  shall also pay to LESSOR as  additional  rent,
LESSEE's  Allocable  Percentage  of any increase in the real estate taxes levied
against  the  Building  and the land on which it is  situated,  above  such real
estate  taxes for fiscal tax year 1997  whether  such  increase  is caused by an
increase in the tax rate,  an increase  in  assessed  value,  or a change in the
method of determining real estate taxes, ("Additional Tax Escalation Rent").

         The base  from  which  the  amount of any  increase  in taxes  shall be
determined from the rate and the assessment for fiscal tax year 1997, or, in the
event LESSOR seeks and its granted an abatement of taxes (administratively or by
appeal to the Appellate  Tax Board and/or the courts) for FY 1997,  the tax rate
and  assessment as abated,  whichever  results in a lower base year amount.  The
Additional Tax Escalation  Rent shall be prorated if the Lease Term commences or
terminates (without breach) during any fiscal year,  accordingly to the fraction
of the total days in such tax year that are included in the Lease Term.

         Notwithstanding  the foregoing,  LESSOR shall be under no obligation to
file for any abatement of taxes for FY 1997 or any other fiscal year, and LESSEE
shall pay the entire amount of Additional Tax Escalation Rent for any fiscal tax
year as invoiced  by LESSOR,  receiving  a rebate of such  payments  made by its
based on its  Allocable  Percentage  of any abated real estate  taxes only if an
abatement is sought and received by LESSOR for such fiscal tax year.

         LESSEE shall make  payment  within  thirty (30) days of written  notice
from LESSOR that Additional Tax Escalation  Rent sums are payable,  which notice
shall be accompanied by a copy of the tax bill.

         5. SECURITY  DEPOSIT.  Upon  execution  hereof,  LESSEE shall post with
LESSOR in the manner described below (and maintain at all times during the Lease
Term and any Extended Term as hereinafter  defined),  a Security  Deposit in the
amount of Two Hundred  Thousand  ($200,000.00)  Dollars,  which shall be held as
security for LESSEE's  performance  as herein  provided,  subject to  reductions
described  below,  with the  balance to be returned to LESSEE at the end of this
Lease Term (as may be  extended);  said 






                                      -6-

reductions and return of any balance subject to LESSEE's satisfactory compliance
with the terms and conditions  hereof.  The Security  Deposit shall be posted at
the  option of the  LESSEE  from time to time (but not  changed  as to form more
frequently than each Lease Year) as follows:

         (a) Two Hundred  Thousand  ($200,000.00)  Dollars by  certified or bank
check (the "Cash Deposit"); or

         (b) Two Hundred Thousand  ($200,000.00) Dollars by irrevocable stand-by
Letter of Credit, drawn on Fleet Bank or another commercial bank having branches
in Massachusetts,  reasonably acceptable to LESSOR, (the "Letter of Credit"). If
Lessee elects to post a Letter of Credit,  the Letter of Credit shall:  (i) name
LESSOR  as  beneficiary;  (ii) be for a term  equal  to the  Lease  Term (or any
extended term, as and when appropriate); (iii) be cancelable only with a minimum
30 days prior  notice to LESSOR;  and (iv)  substantially  in the form  attached
hereto  as  Exhibit  G and in all  respects  in form  and  substance  reasonably
satisfactory  to LESSOR,  which at LESSEE's  election  may  provide  therein for
automatic  direct reduction on the basis of the schedule set forth in subsection
(c) below.

         If LESSOR  so draws  upon the  Letter of Credit or if LESSEE  elects to
post the Cash  Deposit,  LESSOR  shall  deposit  the Cash  Deposit in a separate
account for such  deposits,  with no  administrative  costs or expenses  thereof
passed on to LESSEE,  but interest thereon to inure to the benefit of LESSOR. If
LESSEE has elected to post the Cash  Deposit or LESSOR is holding any portion of
the  Security  Deposit in a form  other  than a Letter of Credit,  and if LESSEE
thereafter  delivers  to LESSOR a Letter of Credit in the amount  and  otherwise
then conforming to the requirements of this Section 5, LESSOR shall promptly pay
over to LESSEE the entire  balance  of the Cash  Deposit or such  portion of the
Security  Deposit  being held by LESSOR,  absent any uncured  default  under the
Lease.  If LESSEE  has  elected  to post the  Letter of  Credit  and  thereafter
delivers to LESSOR the Cash Deposit,  LESSOR shall promptly  return the original
Letter of Credit to LESSEE.  If LESSEE defaults in the payment or performance of
its  obligations  under  this  Lease,  and  such  default  continues  after  any
applicable  notice and the  expiration of any  applicable  grace period,  LESSOR
shall have the right to draw upon and apply the  Security  Deposit to the extent
necessary to cure such default. Within ten (10) days after the expiration of the
Lease Term, as it may be extended, LESSOR shall give written notice to LESSEE of
any defaults by LESSEE in the payment or  performance of its  obligations  under
this Lease.  Within thirty (30) days after the  expiration of the Lease Term, as
it may be extended,  LESSOR shall  return the Security  Deposit to LESSEE,  less
such amounts  necessary  to cure any then  outstanding  defaults  claimed in the
notice to  LESSEE.  If LESSEE  thereafter  within a  reasonable  time  under the
circumstances,  cures said  default,  the Security  Deposit shall be released to
LESSEE.

         (c) Provided  there is not then any  material  default by LESSEE in the
payment or performance of its obligations under this Lease which continues after
notice and the








                                      -7-

expiration of the applicable  cure period,  LESSEE at LESSEE's option may reduce
the Security Deposit according to the following schedule:

Amount of Reduction                                                    Balance
-------------------                                                    -------

End of Lease Year 1                $35,000.00                       $165,000.00
End of Lease Year 2                $35,000.00                       $130,000.00
End of Lease Year 3                $35,000.00                       $ 95,000.00
End of Lease Year 4                $35,000.00                       $ 60,000.00
End of Lease Year 5                N/A                              $ 60,000.00

         6. USE OF  PREMISES.  LESSEE  shall use the Lease  Premises for general
office,  research  and  laboratory  space only,  which uses LESSOR  warrants and
represents  are  currently  allowed under local zoning  regulations  (subject to
compliance with federal,  state and municipal  safety,  healthy,  building,  and
sanitary codes and submission of confirming  plans and  specifications  suitable
for issuance of a building  permit from the City of Cambridge and an appropriate
occupancy  permit upon completion of  construction).  LESSEE will use the Leased
Premises in a careful,  and safe and proper manner and will not do or permit any
act or thing in the Leased  Premises or do any act or thing in or affecting  the
Common Areas which is contrary to any legal or insurance requirement referred to
in Section 17 hereof or which might  impair the value of the Leased  Premises or
the  Building  or any part  thereof or which  constitutes  a risk to the safety,
health or well-being of other tenants in the Building or on the site, or creates
a public or private nuisance or waste.

         7.  UTILITIES.  LESSOR shall  provide at LESSOR's  expense the building
standard facilities for heat,  ventilation,  and air conditioning for the Leased
Premises,  and the common areas and facilities  which LESSEE enjoys the right to
use, as required for  comfortable  occupancy,  during 8 AM to 6 PM each weekday,
other than national or state holidays  (herein  "Normal  Business  Hours").  Any
change in such hours by LESSOR for the Building generally shall inure as well to
the benefit of the LESSEE.  LESSOR shall provide  electrical  connections to the
Leased Premises for general office, research, and laboratory purposes,  pursuant
to its build-out obligations set forth on Exhibit C hereto. If any operations or
activities  in  the  Leased  Premises  use  electricity  in  excess  of  amounts
customarily  required for office purposes,  LESSOR, in its discretion and at its
expense, may install submeters to measure such usage separately. Notwithstanding
any separate  direct metering or general  allocation,  LESSEE shall pay all such
charges for  electricity  used on the Leased  Premises  as it may be  separately
metered,  or absent  separate meters to the whole or part of the Leased Premises
(for  whatever  reason)  based on  tenant's  Allocable  Percentage  of the total
electric bill,  whichever or both as may be applicable,  at the determination of
the LESSOR.  LESSOR shall  determine  said  electrical  charges in a uniform and
non-discriminatory  manner  relative to other lessees in the Building  similarly
situated;  and LESSEE shall pay its proportional  share of said charges relative
to all  occupied  space which is not billed to tenants on a  separately  metered
basis.  All such charges  shall be based on the rates at which LESSOR is charged
for electricity  supplied to











                                      -8-

the  Building  by  the  utility  company.   LESSOR  shall  maintain  an  average
temperature  in the  Building  between  60  degrees  Fahrenheit  and 80  degrees
Fahrenheit  at all times  and an  average  temperature  in the  Leased  Premises
generally between 68 degrees  Fahrenheit and 76 degrees Fahrenheit during Normal
Business   Hours.   LESSOR  shall  make   available   heat,   ventilation,   and
air-conditioning  as may be requested by LESSEE for the Leased  Premises  during
hours  other than Normal  Business  Hours  ("Overtime  HVAC"),  and  (subject to
increase by the same percentage  amount by which the standard electric rates are
increased from time to time by the utility company),  as billed by LESSOR and to
be paid by LESSEE  within  thirty (30) days of said  invoice.  LESSEE shall give
LESSOR  24 hours  prior  notice of any  requirements  for  specialized  overtime
heating  and air  conditioning.  LESSOR  shall not be  liable to LESSEE  for any
interruption,  interference,  damage or loss to LESSEE's  business,  research or
experimentation  occasioned  as a result of any failure or  interruption  in the
heating,   ventilation,  air  conditioning,  or  electrical  services  or  other
utilities  servicing the Building or the Leased  Premises;  unless caused by the
willful  misconduct or negligence of LESSOR;  in which case all  installments of
Annual  Base Rent and  Additional  Rent  shall be abated  based on the amount of
space  within  the  Leased  premises  which is not  available  to LESSEE for its
business  activities,  if the failure or  interruption  continues  for more than
three (3) days after  notice to LESSOR;  and in which case  LESSEE  shall have a
right to terminate this Lease if said failure  resulting  from LESSOR's  willful
misconduct or negligence continues for more than sixty (60) days after notice to
LESSOR).  No  plumbing  or  electrical  work of any type  shall be done  without
LESSOR's  approval which approval shall not be unreasonably  withheld or delayed
and the  appropriate  municipal  permit and inspector's  approval.  Hot and cold
water for domestic type sanitary  purposes (only) shall be supplied by LESSOR at
LESSOR's expense.  LESSOR shall also supply working  connections for non-potable
laboratory  water  and  water  for  other  particularized  uses  in the  Demised
Premises; usage to be separately metered and paid for by LESSEE; and all charges
for such  separately  metered  water  used by  LESSEE to be based on the rate at
which water/sewer charges are billed to the Building by the City of Cambridge.

         8. COMPLIANCE WITH LAWS. LESSEE acknowledges that no trade, occupation,
or activity shall be conducted in the Leased  Premises or use made thereof which
will be unlawful, noisy or offensive, or contrary to any federal or state law or
administrative  regulations,  or any municipal ordinance or regulations in force
at any time in Cambridge.  LESSEE shall keep all employees working in the Leased
Premises covered with Worker's  Compensation  Insurance,  as applicable.  LESSEE
shall be responsible for complying with the  Occupational  Safety and Health Act
of 1970 and any  amendments  thereto,  in  connection  with  LESSEE's use of the
Leased Premises. LESSEE shall strictly adhere to any and all federal, state, and
municipal  laws,  ordinances,  and  regulations  governing  LESSEE's  laboratory
scientific experimentation. LESSEE shall be solely responsible for procuring and
complying at all times with any and all necessary  permits directly  relating or
incident to: the conduct of its office and research  activities on the Premises;
its scientific experimentation; and LESSEE's transportation,  storage, handling,
use and disposal of any low level radioactive or bacteriological or pathological
substances or organisms or other hazardous wastes or  environmentally  dangerous
substances  or











                                      -9-

materials  to, from or at the Leased  Premises.  LESSEE shall  immediately  give
notice to LESSOR of any warnings or  violations  relative to the above  received
from any federal,  state, or municipal  agency or by any court of law, and shall
immediately begin and diligently proceed to cure the conditions causing any such
violations;  and LESSOR shall permit LESSEE to cure said harm or hazard prior to
any  active   intervention   by  LESSOR  (except  where  such   intervention  is
necessitated by the emergency nature or the harm or hazard; or where the harm or
hazard  impairs the value of the  Building,  (directly or as  collateral  on any
debt);  interferes  with  any  other  tenant's  rights;  or is  required  by any
governmental agency or authority.

         LESSEE shall fully  indemnify and hold  harmless in al respects  LESSOR
from any and all claims, demands,  losses,  liabilities,  and damages (including
all   necessary   and   reasonable   expenses  for   contractors,   consultants,
environmental  engineers,  attorneys, and other professionals utilized by LESSOR
to evaluate  and  remediate  any hazard or harm which  LESSEE has failed to cure
after  written  notice from LESSOR;  and further  including any and all fines or
fees assessed by any  governmental  agency relative to any hazard or harm to the
extent  directly  arising  from the conduct of  LESSEE's  research on the Leased
Premises  (especially  relating to research  involving  hazardous  substances),)
resulting  from  LESSEE's  obligations  and  responsibilities  with  respect  to
compliance with environmental  laws and regulations and LESSEE'S  obligations to
cure,  as set forth above and herein,  except for any claims,  demands,  losses,
liabilities  and damages  resulting from the acts or negligence of LESSOR or its
agents or employees or independent contractors.

         LESSOR hereby  represents  and warrants to LESSEE that the Building and
its heating,  ventilation, air conditioning,  plumbing,  electrical, life safety
and other mechanical  systems and equipment  comply with all applicable  federal
state and municipal laws, ordinances, and regulations. LESSOR shall maintain the
Building  and such  systems  and  equipment  in  compliance  with all such laws,
ordinances and  regulations,  except to the extent governing the use of areas or
premises controlled by LESSEE or any other tenants in the Building.

         LESSOR  hereby  represents  and  warrants  to  LESSEE  that the  Leased
Premises  and the  building of which they are a part,  (inclusive  of the Common
Areas) are, to the best of LESSOR's knowledge, in compliance with all applicable
building,  sanitary  and zoning laws,  ordinances  and  regulations,  (including
current  applicable  provisions of the Americans  with  Disabilities  Act);  and
LESSOR has not been cited with any current and outstanding  violations  thereof.
Additionally,  LESSOR  hereby  represents  and  warrants  relative to the Leased
Premises and the building of which they are a part,  (inclusive of Common Areas)
that it has received no  remediation  notices  pertaining  to, nor are there any
outstanding  violations to the best of its knowledge of, the  provisions of G.L.
c.21E  or  the  Massachusetts  Contingency  Plan;  or  any  other  environmental
remediation statutes or regulations.

         9. FIRE AND OTHER  INSURANCE.  LESSEE  shall not  permit any use of the
Leased Premises which will make voidable,  increase any premium, or decrease any













                                      -10-

insurance on the Building and property of which the Leased  Premises are a part,
or on the  contents  of said  Building,  or which  shall be contrary to any law,
regulation,  or order from time to time  established or issued by the local Fire
Department, or any similar body, or any restriction contained in any of LESSOR'S
insurance  policies as to the Building and property;  provided  however,  LESSOR
represents and warrants that LESSOR's  insurance  policies shall not contain any
restrictions or LESSEE's use of the Premises for office, research and laboratory
purposes  and such use,  per se,,  shall not  render  such  insurance  voidable,
increase any premium,  or decrease any of LESSOR's  coverages.  LESSEE shall, on
demand,  reimburse LESSOR all extra insurance premiums caused by LESSEE's use of
the Leased Premises other than for the office,  research and laboratory purposes
contemplated herein.  LESSEE shall not vacate the Leased Premises or permit same
to be  unoccupied  for more than  thirty  (30) days other than  during  LESSEE's
customary non-business days or hours.

         10.   MAINTENANCE   OBLIGATIONS.   LESSOR  shall   perform  all  normal
maintenance,  repairs and  replacements  necessary to keep in good condition and
working order (a) the roof, foundation,  structural columns and other structural
elements  of the  Building,  (b) the  heating,  ventilation,  air  conditioning,
plumbing,  electrical,  life safety and other  mechanical  systems and equipment
servicing  the  Building or the Common  Areas,  (c) the parking  areas,  (d) the
driveways and walkways  necessary for access to the Building and parking  areas,
(e) the entrances,  lobbies, stairs, passenger elevators and corridors necessary
for access to the Leased Premises,  (f) the loading docks and freight  elevators
in the Building,  (g) the  lavatories  on the fourth  floor,  and (h) the Common
Areas,  generally.  Notwithstanding  the  foregoing,  any  damage  caused to the
building or any of the aforesaid  components caused by the careless,  malicious,
willful, or negligent acts of LESSEE; and chemical, water or corrosion damage on
or  emanating  from the Leased  Premises  from any source  within the control of
LESSEE;  shall be repaired by LESSOR and specifically and separately assessed as
against LESSEE.  LESSEE agrees to maintain at its expense all other elements and
components  to the  Leased  Premises  in the same  condition  as they are at the
commencement of the Term or as they may be put in during the Term of this lease,
normal  wear and tear and  damage by fire or  casualty  or eminent  domain  only
excepted,  and whenever  necessary,  to replace light bulbs (after the first six
months of the term),  plate  glass and other  glass  therein,  acknowledging  by
acceptance of the delivered  Leased Premises as of the Phase I Delivery Date and
Phase II Delivery Date that the Leased  Premises upon delivery are in good order
(except for punchlist items, which are to be completed by LESSOR as contemplated
in  paragraph  33  hereof),  and the light  bulbs and glass  whole.  LESSEE will
properly  control or vent all solvents,  degreasers,  and the like and shall not
cause the area  surrounding  the Leased  Premises to be in anything other than a
neat and  clean  condition,  depositing  all waste in  appropriate  receptacles.
LESSEE shall not permit the Leased Premises to be overloaded,  damaged, stripped
or defaced,  suffer any waste of the Leased Premises.  Any maintenance  which is
the  responsibility  of LESSOR and which is necessitated by some specific aspect
of  LESSEE's  negligent  or  reckless  use of the  Leased  Premises  shall be at
LESSEE's  expense,  except to the extent LESSOR receives  reimbursement  for any
such costs or expenses from an insurer. All maintenance provided by LESSOR shall
be performed as reasonably required at 












                                      -11-

LESSOR's  reasonable  discretion  and except for  emergencies,  during  LESSOR's
normal  business  hours.  LESSEE may not keep any animals on the Leased Premises
without prior written notice to the approval from LESSOR in each instance, which
approval may be denied or conditioned in LESSOR's reasonable discretion.  LESSEE
shall be solely  responsible for maintenance and operation of any and all of its
systems  installed  by or for  the  LESSEE  or  servicing  the  Leased  Premises
exclusively,  and shall waive any and all claims  against LESSOR for any damage,
impairment, or loss relative to these systems unless caused by the negligence or
willful misconduct of LESSOR, its agents, employees and independent contractors.
Specifically,  LESSEE shall maintain,  at its sole expense,  and pay all charges
for  electrical  service  (according to section 7 of this Lease) and use of, the
following:  (a) LESSEE's  customized "cold room" or "warm room" (if any) and all
equipment associated with its operation,  and (b) additional HVAC system (to the
extent of the customization of the LESSOR's  building standard HVAC system;  (c)
backflow preventers (other than the Building's  standard equipment  incorporated
in  the  Building's  general  plumbing  and  life  safety  systems);   (d)  acid
neutralization chip tanks; and (e) any other specialized equipment or mechanical
systems serving the Leased Premises exclusively.

         11.  ALTERATIONS.  LESSEE  shall  not make  structural  alterations  or
additions  of any  kind  to the  Leased  Premises,  but may  make  nonstructural
alterations provided LESSOR consents thereto in writing,  said consent not to be
unreasonably  withheld  or  delayed.  Notwithstanding  the  foregoing,  LESSOR's
consent shall not be required for any non-structural alterations or additions to
the  Leased  Premises  which (a) do not affect the  structural  elements  of the
Building, or the Building's heating, ventilation, or air conditioning, plumbing,
mechanical,  electrical, elevator, life safety or other common systems or Common
Areas of the  Building,  and (b) do not cost more than  $10,000.00 in any single
instance.  Plans and  specifications  shall be  submitted by LESSEE to LESSOR in
each  instance  where consent is required,  in advance of any proposed  work, in
sufficient detail and scope to enable LESSOR to make a reasonable  determination
thereon.  All such allowed alterations shall be at LESSEE's expense and shall be
in quality at least equal to the present  construction.  If LESSOR  performs any
services  for LESSEE in  connection  with such  alterations  or  otherwise,  any
invoice  therefore will be promptly paid.  LESSEE shall not permit any mechanics
liens,  or similar liens,  to remain upon the Leased Premises in connection with
work of any  character  performed  or  claimed  to have  been  performed  at the
direction  of LESSEE and shall  cause any such lien to be  released,  removed or
bonded without cost to LESSOR,  within twenty (20) days after  completion of the
work giving rise to such lien.  LESSEE shall not permit any changes for labor or
materials  secured by any such liens to remain  unpaid for more than thirty (30)
days after  receiving  notice of such charges unless LESSEE protects LESSOR from
any such liens by bond or other  assurances  reasonably  satisfactory to LESSOR.
Any  alterations  completed by LESSEE,  including,  without  limitation,  window
blinds or other  window  treatment,  shall be building  standard  unless  LESSOR
expressly agrees otherwise,  which agreement shall not be unreasonably  withheld
or delayed. LESSOR shall have the right at any time to change the arrangement of
parking areas,  stairs,  walkways or other common areas of the








                                      -12-

Building of which the Leased  Premises are a part,  provided such changes do not
interfere with LESSEE's use or access to such areas and facilities.

         Notwithstanding  the foregoing,  prior to the  commencement of the Term
hereof,  and pursuant to Section 33, LESSOR shall, at its sole cost and expense,
deliver the Leased Premises "as is" but  nevertheless  such that they conform to
LESSOR'S  standard  Building  specifications,  except that  LESSOR  specifically
agrees that prior to its delivery of the Premises to the LESSEE, LESSOR shall at
its sole cost and expense: Repaint the Premises; repair/replace carpeting as and
where necessary based on extraordinary  current wear; and prepare LESSOR's basic
and standard  infrastructure  for LESSEE's  laboratory  space,  and otherwise to
construct  and install the  alterations  contained  within  LESSOR's Work as set
forth more  specifically  in section 33 of this Lease.  The space  delivered  to
LESSEE  shall be as set  forth on the  space  plan and  specifications  attached
hereto as Exhibit C. All laboratory  equipment,  (not  specifically  included in
LESSOR's Work as contemplated by paragraph 33 hereof), including but not limited
to hoods,  vacuum pumps and RODI water system(s) shall be provided and installed
at LESSEE's sole cost and expense.

         12.  ASSIGNMENT,  SUBLETTING.  LESSEE covenants and agrees that neither
this Lease,  nor the estate  hereby  granted,  nor any interest  therein will be
assigned,  mortgaged,  pledged,  encumbered or otherwise  transferred,  and that
neither the Leased  Premises,  nor any part  thereof,  will be encumbered in any
manner by reason or by act or omission (where LESSEE has a legal duty to act) of
LESSEE,  or used or occupied,  or  permitted  to be used or occupied,  by anyone
other than LESSEE, its servants, agents and employees, or for any use or purpose
other than as above stated,  or be sublet,  without in each case LESSOR's  prior
written consent, which shall not be unreasonably  withheld.  Notwithstanding the
foregoing, LESSEE shall have the right, without the prior consent of the LESSOR,
to assign  this Lease and to sublet any  portion of the Leased  Premises  to any
person or entity (a)  controlling,  controlled  by, or under common control with
LESSEE,  (b) acquiring all or substantially  all of the assets of the LESSEE, or
(c) with or into which LESSEE merges or consolidates;  provided in each instance
the LESSEE shall remain at all times  directly,  primarily and severally  liable
for the  performance of all terms and  conditions of this Lease,  in conjunction
with any other such entity.

         The grounds upon which LESSOR may  reasonably  withhold its consent are
as follows:

         (i) The  prospective  assignee's  or  sublessee's  intended  use of the
Premises is not identical to the permitted uses set forth in the Lease; or,

         (ii) The nature,  character,  class and  standards  of the  prospective
assignee's or  sublessee's  business will not be consistent  with those of other
lessees in the  Building or will not conform to the mix of other  lessees in the
Building at that time; or,







                                      -13-

         (iii) The  financial  net worth,  and  reliability  of the  prospective
assignee or sublessee,  including any additional  written and direct personal or
corporate  guarantees,  is  not  in  Landlord's  reasonable  judgment  at  least
equivalent  to that of DynaGen,  Inc. as measured at the point of  execution  of
this Lease;  (provided however that if the proposed  assignment or sublet occurs
during the  Extended  Term (i.e.  after the original  five (5) years),  then the
financial net worth of the prospective assignee or sublessee shall be equivalent
to that of DynaGen,  Inc. as measured at the point of  execution  of this Lease,
but said net worth  standard  to be  adjusted by  increasing  by the  cumulative
increases in the Consumer Price Index, All Items, All Urban,  occurring from the
date of this Lease to the date of the proposed  assignment or  subletting).  The
prospective  assignee  or  sub-lessee  must  produce to LESSOR if  available,  a
verified and current audited financial  statement  prepared within the then past
three years,  (or if non has been prepared by said  prospective  assignee within
the then past three years, a CPA certified  current  financial  statement);  and
such  other   reasonable   documentation   as  is  material  in  making  such  a
determination; or,

         (iv) The  operations of the  prospective  assignee or  sub-lessee  will
violate any  exclusive or other rights given any other  lessees in the Building;
or,

         (v) The failure of LESSOR's mortgage lender(s) to consent.

         LESSOR,  in addition to Annual Base Rent and Additional  Rent, shall be
entitled to the full amount of any and all sums  assessed or collected by LESSEE
in whatever form,  attributable  to the assignment or sublease or other transfer
of LESSEE's interest in this Lease or the Leased Premises (except for a sublease
under clause (ii) below),  which exceed said Annual Base Rent or Additional Rent
hereunder,  (therein,  "Rent Mark-Up"),  after deduction of LESSEE's  reasonable
costs and expenses to procure said assignment or sublet, including brokers' fees
and commissions,  attorneys' fees, and any build-out costs (approved by Landlord
in advance,  said approval not to be  unreasonably  withheld or delayed),  which
costs and expenses shall in no event be deducted from any payments to LESSOR due
in the form of Annual Base Rent,  Additional Operating Expense Rent,  Additional
Tax Escalation Rent, or other sums payable to LESSOR under this Lease other than
the aforesaid excess amount on assignment or subletting.

         Notwithstanding  any LESSOR's  consent to any assignment or subletting,
as contemplated  above, or in any circumstances  other than a LESSOR's recapture
of the entire  premises as  contemplated  below,  LESSEE shall remain  primarily
liable to LESSOR  for the  payment  of all  Annual  Base Rent and all other Rent
hereunder,  and for the full performance of the covenants and conditions of this
Lease to be performed  by LESSEE;  and after a default by LESSEE in such payment
or performance  which  continues  after any  applicable  notice and cure period,
LESSOR may collect all sums due as Annual Base Rent or other Rent  directly from
the assignee or subtenant.








                                      -14-

         Notwithstanding  the  foregoing,  in the event that  LESSEE  desires to
sublet any portion of the Leased Premises, then the following shall apply:

         (i) if the proposed  sublet is on the basis where a demising wall is to
be erected by LESSEE or the  subtenant,  separating  their spaces in whole or in
part,  pursuant to an agreement with a bona fide third party  subtenant,  at any
time during the sublet, then LESSOR shall be granted its Recapture Rights as set
forth below as the portion of the Leased Premises to be so sublet; or,

         (ii) if the  proposed  sublet is on the basis  where  there  will be no
demising  wall  erected,  separating  their spaces in whole or in part,  and the
subtenant  is  reasonably  expected  to utilize up to (but not more than)  fifty
(50%) percent of the facilities  comprising the Leased  Premises,  then LESSOR's
Recapture Rights shall not apply; or,

         (iii) if the  proposed  sublet is on the basis  where  there will be no
demising  wall  erected,  separating  their spaces in whole or in part,  and the
subtenant is reasonably expected to utilize more than fifty (50%) percent of the
facilities comprising the Leased Premises,  then LESSOR's Recapture rights shall
apply as to the entire Leased Premises.

         In all  instances  set forth above where  reference is made to LESSOR's
Recapture Rights,  those rights shall consist of the following:  LESSEE, in each
instance where  Landlord's  Recapture  Rights apply,  shall notify the LESSOR in
writing,  stating its  intention  to sublet and the target date of the  proposed
sublet (which shall not be less than one hundred twenty (120) days from the date
of said notice to LESSOR).  LESSOR  shall have a period of ninety (90) days from
the date it receives  such notice to  exercise  an  election to  recapture  that
portion of the Leased  Premises to be sublet,  in LESSOR's sole  discretion  and
without  any   obligation   to  so  elect,   whatsoever,   notwithstanding   the
circumstances,  and without  prejudice to or waiver of any of LESSOR's rights or
LESSEE's continuing obligations hereunder.  LESSEE shall provide LESSOR with all
material  information  relative to LESSOR making an informed decision concerning
said sublet,  immediately upon LESSOR's  request.  If LESSOR elects to recapture
the Leased  Premises  (or  applicable  portion  thereof),  it shall send written
notice thereof to LESSEE; and LESSEE shall be irrevocably bound to surrender and
vacate the Leased Premises (or applicable  portion thereof) as if the Lease Term
had expired on the date set forth in the LESSEE's initial notice to LESSOR;  and
provided LESSEE vacates and surrenders on said date, without being in default of
any  provision  hereof as of said date,  this  Lease  shall be null and void and
without   recourse  to  either  party  hereto  (but  for  terms  and  conditions
contemplated herein to survive  termination of this Lease).  LESSEE shall not be
entitled to any payments,  commissions,  credits, offsets, or any kind or nature
arising from said sublet, nor shall any individual or entity acting by, through,
or under LESSEE be so entitled. Once an election to recapture is made by LESSOR,
LESSEE  shall be subject to the  penalties  for  holding  over set forth in this
Lease,  if it fails to vacate and surrender the Leased  Premises (or  applicable
portion  thereof) by the date stated in the notice,  or if it fails to discharge
(or cause its lenders or others with which  LESSEE has dealt to  discharge)  any
and all liens or other  encumbrances,  notices, or restrictions on its leasehold








                                      -15-


or contractual  interest in and to the Leased Premises as of said date.  Nothing
in this  section  or  paragraph  shall  require  LESSOR to make an  election  to
recapture the Leased Premises (or applicable  portion  thereof),  and nothing in
the aforesaid  process shall  relieve  LESSEE of its inability  under this Lease
should LESSOR elect not to take back the Leased Premises (or applicable  portion
thereof).  Notwithstanding  the foregoing,  LESSEE shall have ten (10) days from
LESSOR's notice of its election to recapture, to withdraw its proposal to sublet
in writing delivered to LESSOR,  whereupon the LESSOR's election shall be deemed
rescinded and LESSEE shall remain in the Leased Premises, without sublet. LESSEE
may only  exercise  this  withdrawal  right a maximum of once  within each Lease
Year.  Should  LESSOR elect not to recapture  the Leased  Premises,  then LESSEE
shall have a period of one hundred  twenty  (120) days from the date of LESSOR's
notice of such election (or if LESSOR  delivers no notice of any election,  then
one hundred  twenty (120) days from the  expiration of LESSOR's  ninety (90) day
response period) to execute its sublease  agreement and commence the subtenancy,
without the necessity for any  additional  notices to LESSOR under this section.
New  notice by  LESSEE to LESSOR  shall be  required  after  expiration  of said
one-hundred  twenty (120) day period.  Time is of the essence in all notices and
elections made hereunder.

         13.  SUBORDINATION.  This Lease shall be subject and subordinate to any
and all instruments of record, mortgages, and other instruments in the nature of
a mortgage,  extant or coming into existence at any time  hereafter,  and LESSEE
shall, when requested, promptly within fifteen (15) days of request, execute and
deliver  such  written  instruments  (on  LESSOR's  lender's  form)  as shall be
necessary to show the subordination of this Lease to said instruments of record,
mortgages, or other such instruments in the nature of a mortgage.  LESSOR, prior
to the Phase I Delivery Date and within 21 days of any refinancing or additional
financing  of  the  building  and/or  property,  shall  provide  LESSEE  with  a
Non-Disturbance,  Attornment and Subordination Agreement, recognizing the rights
of LESSEE under this Lease. LESSEE acknowledges that an agreement  substantially
in  the  form  attached  hereto  as  Exhibit  G,  shall  satisfy  the  foregoing
requirements.

         14. LESSOR'S ACCESS. LESSOR or agents of LESSOR may at reasonable times
and upon advance  reasonable  notice (except in emergency  situations)  enter to
view the Leased  Premises  and may remove any signs not  approved and affixed as
herein provided,  and may make repairs and alterations as LESSOR should elect to
do and repairs  which  LESSEE is  required  but has failed to do (but only after
notice  and an  opportunity  to  repair  being  provided  to LESSEE  within  the
applicable  cure period under this Lease),  and may show the Leased  Premises to
prospective mortgagees,  appraisers, brokers, prospective purchasers, and within
twelve months of the expiration of the Term or on any default other  prospective
tenants.  Additionally,  to the extent  necessary to service other  portions the
Premises or the Common Areas or other tenant spaces in the Building;  LESSOR may
add,  relocate,  or  maintain a chase,  pipes,  conduits,  or ducts,  within the
Premises provided the aforesaid do not materially interfere with LESSEE's use of
the Premises or its aesthetics.  Any entry by LESSOR,  its agents,  employees or
independent contractors onto the Premises under this section 14 shall be done in
such manner as to 








                                      -16-

minimally   interfere  with  the  business  conducted  thereon  by  LESSEE,  and
undertaken with reasonable steps to protect LESSEE's property.

         15. SNOW REMOVAL. LESSOR, at its sole expense, shall keep the walkways,
sidewalks,  entry  ways  and  parking  areas  reasonably  clear of snow and ice.
Notwithstanding the foregoing,  however,  LESSEE shall hold LESSOR harmless from
any and all claims by LESSEE's  agents,  representatives,  employees or business
invitees for damage or personal injury  resulting in any way from snow or ice on
any area servicing the Building, unless such claims arise from the negligence or
willful   misconduct  of  LESSOR  or  its  agents,   employees  or   independent
contractors.

         16. ACCESS AND PARKING.  LESSEE shall be granted the right,  at current
rates (which may be increased form time to time to reflect market increases,  to
park twelve (12) cars in the  building's  on-site indoor parking lot or facility
in  single or  tandem  spaces,  or on a valet  basis,  which  LESSOR in its sole
discretion  shall designate from time to time; (at all times hereunder LESSEE to
be entitled to four single  spaces from among those  twelve).  LESSOR  initially
designates  an  appropriate  proportion of the same single and tandem area as is
used by the lessee  presently  vacating the  Building,  as the LESSEE's  initial
parking area. The initial parking rate therefore shall be $100.00 per month, per
car,  which monthly rate shall be subject to and  reflective of periodic  market
changes.  Additionally,  LESSEE  shall be  entitled  to twelve  (12)  additional
parking  spaces in the  garage  (not  owned by  LESSOR)  at 808  Memorial  Drive
Cambridge,  so long as they are  available  to  LESSOR  during  the Term of this
Lease,  at an initial  parking rate of $90.00 per month,  per car, which monthly
rate shall be subject to and  reflective  of  periodic  market  changes and upon
thirty (30) days prior written notice to LESSEE.  Said garage, lot, or facility,
as the case may be,  plus any  stairs,  walkways  or other  means of  ingress or
egress  controlled by the Lessor shall not in any case be considered  extensions
on the Leased  Premises.  LESSEE will not  obstruct in any manner any portion of
the Building or the walkways or approaches to the Building,  and will conform to
all reasonable and non-discriminatory  rules now or hereafter made by LESSOR for
parking,  (but which rules shall in no event limit  LESSEE's right to its twelve
(12) indoor parking spaces), and for the access and egress, security, care, use,
or alteration of the Building,  its  facilities and  approaches.  LESSEE further
warrants  that LESSEE will not permit any employee or visitor to violate this or
any other covenant or obligation of LESSEE.  No vehicles shall be stored or left
in any  parking  area  for more  than  three  nights  without  LESSOR's  written
approval.  Unregistered or disabled  vehicles,  or storage trailers of any type,
may not be parked  overnight  any time.  LESSEE agrees to assume all expense and
risk for the towing of any  misparked  vehicle  belonging  to LESSEE or LESSEE's
agents,  employees,  business invitees, or callers, at any time. For the purpose
of this  section  the term  "space"  shall  mean  general  access  for one motor
vehicle.  All vehicles shall be parked and left on the premises at their owners'
sole risk and LESSOR shall not be liable for any damages caused to said vehicles
while they are parked or left on the premises.

         Additionally,  both parties  acknowledging that the garage is currently
full,  LESSOR shall make reasonable  efforts to provide an additional  number of
parking spaces within the 








                                      -17-

Building's  garage, as such spaces may become available in the future;  however,
LESSOR shall not be obligated to violate any other  exclusive  rights granted in
existing or future leases.

         17. LESSEE'S AND LESSOR'S LIABILITY  INSURANCE.  LESSEE shall be solely
responsible as between LESSOR and LESSEE for deaths or personal  injuries to all
persons  whomsoever  caused  or  occurring  in or on the  Leased  Premises  from
whatever  cause  arising,  (unless  caused by the negligent acts or omissions or
willful   misconduct  of  LESSOR  or  its   employees,   agents  or  independent
contractors), and damage to property to whomsoever belonging caused or occurring
in or on the Leased  Premises  to the extent  arising  out of the use,  control,
condition or occupation of the Leased  Premises by LESSEE  (except to the extent
such damage arises out of the negligence or willful  misconduct of LESSOR or its
employees,  agents, or independent contractors);  and LESSEE agrees to indemnify
and save  harmless  LESSOR  from  any and all  liability,  reasonable  expenses,
damage,  causes of action, suits, claims or judgments caused by any such deaths,
personal  injuries  or  damages  to  property,  except to  extent  caused by the
negligence or willful misconduct of LESSOR, its employees, agents or independent
contractors.  During  the Lease  Term  LESSEE  will  secure and carry at its own
expense a  comprehensive  general  liability  policy insuring LESSEE against any
claims based on bodily injury  (including death) arising out of the condition of
the Leased Premises or their use by LESSEE, such policy to insure LESSEE against
any claim up to One Million  ($1,000,000.) Dollars for personal injury or damage
to property.  LESSOR and its lenders (of which LESSEE is given  written  notice)
shall be included in such policy as additional insureds,  as their interests may
appear,  from time to time.  LESSEE will promptly file with LESSOR  certificates
showing  that such  insurance  is in force,  and  thereafter  will file  renewal
certificates  prior to the expiration of any such  policies.  All such insurance
certificates  shall provide that such policies shall not be canceled  without at
least thirty (30) days prior written notice to each insured named therein.

         During  the Lease Term  LESSOR  shall  secure and  maintain a policy of
commercial general liability insurance covering LESSOR on an occurrence basis in
an  amount  not less  than  $1,000,000.00  for  claims  based on  bodily  injury
(including death), personal injury, and property damage relating to the Building
and the  property on which the  building is located;  and a policy of  insurance
covering  the  building  and other  improvements  on the  property  on which the
building is located for direct risk of physical loss on an occurrence  basis, in
an  amount  equal to the  replacement  costs  of the  Building  and  such  other
improvements  (exclusive  of tenant  improvements  owned by such  tenants) in at
least the minimums required by LESSOR's lendors.

         LESSOR and LESSEE  covenant that with respect to any  insurance  policy
required hereunder to be carried by either, such insurance shall provide for the
waiver by the insurance  carrier of any  subrogation  rights  against the other;
where such  waiver of  subrogation  rights  does not  require  the payment of an
additional  premium or, if an  additional  premium is required,  the other party
pays such premium  within a reasonable  time after written  notice thereof if it
elects to have the benefits of such protection.









                                      -18-

         18. FIRE, CASUALTY, EMINENT DOMAIN. Should a substantial portion of the
Leased  Premises,  or of the property of which they are a part, be substantially
damaged by fire or other  casualty,  or be taken by eminent  domain,  LESSOR may
elect to terminate  this Lease by written  notice to LESSEE  within  ninety (90)
days after such damage occurs,  provided LESSOR  terminates the leases for other
tenants in the  Building,  similarly  affected by such  damage.  When such fire,
casualty,  or taking renders the Leased  Premises  substantially  unsuitable for
their  intended use and no  termination  has been elected by LESSOR,  a just and
proportionate abatement of rent shall be made from the date of such damage until
repairs  have been  substantially  completed,  and LESSEE may elect to terminate
this Lease if: (a) LESSOR fails to give written  notice  within ninety (90) days
of  intention  to restore  Leased  Premises,  or (b) LESSOR fails to restore the
Leased  Premises to a condition  substantially  suitable for their  intended use
within one hundred  eighty (180) days of said fire,  casualty or taking.  LESSOR
reserves  all rights for all  damages or injury to the Leased  Premises  for any
taking by  eminent  domain;  except for damage to  LESSEE's  moveable  fixtures,
property or equipment,  or moving expenses,  which are specifically allocated to
LESSEE by the taking authority or arbitrators.

         19.  BROKERAGE.  LESSEE and LESSOR each warrants and  represents to the
other that they have dealt with no broker or third  person with  respect to this
Lease or the Leased Premises or Building entitled to a commission as a result of
this Lease  other than The Abbey Corp and  Fallen  Hines &  O'Connor,  whose fee
shall be paid by LESSOR;  and LESSOR and LESSEE each agree to indemnify and hold
harmless the other from any fees,  expenses,  or damages  arising from breach of
the above warranty.

         20. SIGNS. LESSEE shall have the right to have its name included on any
central  directory  maintained by LESSOR listing the  building's  other tenants.
LESSOR  authorizes  LESSEE,  if  desired,  to display  one sign  (which may bear
LESSEE's logo) on LESSEE's office entrance door consistent with similar signs of
other tenants. LESSEE shall obtain the written consent of LESSOR before erecting
any sign on the Leased Premises,  which consent may be conditioned on compliance
with  LESSOR's  requests as to size,  wording,  and location of such signs,  but
which shall not be unreasonably withheld or delayed.

         21.  DEFAULT,  BANKRUPTCY AND  ACCELERATION OF RENT. In the event that:
(a)  LESSEE  shall  default  in the  payment  of  the  security  deposit  or any
installment  of Annual Base Rent or any  Additional  Operating  Expense  Rent or
Additional  Tax  Escalation  Rent or other  Rent or other  payments  due  LESSOR
hereunder,  and such  default  shall  continue  for ten (10) days after  written
notice thereof;  or (b) LESSEE shall default in the observance or performance of
any other of LESSEE's covenants,  agreements,  or obligations hereunder and such
default  shall not be corrected  within  thirty (30) days after  written  notice
thereof,  or such longer  period as is necessary to  effectuate a cure  provided
LESSEE  promptly  proceeds to cure and  diligently  pursues such  resolution  to
completion,  but not longer than ninety (90) days;  (c) LESSEE shall be declared
bankrupt or  insolvent  according  to law, or if any  voluntary  or  involuntary
petition 






                                      -19-

for bankruptcy is filed against  LESSEE and not  discharged  within 60 days from
filing;  or if any assignment shall be made of LESSEE's property for the benefit
of creditors;  then, while such default continues, and without demand or further
notice,  LESSOR shall have the right to re-enter and take complete possession of
the Leased  Premises,  to declare  the term of this Lease  ended,  and to remove
LESSEE's  effects,  without  being  guilty of any manner of trespass and without
prejudice to any remedies  which might be otherwise  used for arrears of rent or
other default or breach of covenant.  LESSEE shall indemnify  LESSOR against all
loss of Annual Base Rent,  Additional Operating Expense Rent, and Additional Tax
Escalation  Rent and all other payments which LESSOR may incur by reason of such
termination  during  the  remainder  of  the  Lease  Term,  it  being  expressly
understood that LESSOR shall use reasonable efforts to relet the Leased Premises
and  collect  all rents from such  reletting.  If LESSEE  shall  default,  after
reasonable notice thereof, in the observance or performance of any conditions or
covenants on LESSEE's part to be observed or performed under or by virtue of any
one of the provisions in any section of this Lease, LESSOR,  without being under
any  obligation to do so and without  thereby  waiving such  default,  may after
notice to LESSEE and the expiration of any applicable  cure period,  remedy same
for the  account  and at the  expense of LESSEE  (including  but not  limited to
application  of any or all of the Security  Deposit held by LESSOR in accordance
with section 5 of this Lease).  If LESSOR pays or incurs any obligations for the
payment  of  money  in  connection  therewith,  including  but  not  limited  to
reasonable  attorney's fees in instituting,  prosecuting or defending any action
or proceeding, such sums paid or obligations incurred, with interest at the rate
of fifteen  percent  per annum and  costs,  shall be paid to LESSOR by LESSEE as
additional  rent. Upon default of this Lease by LESSEE,  and because the payment
of Rent in monthly  installments  is for the sole  convenience  of  LESSEE,  the
entire  balance  of Rent which  would  accrue  hereunder  shall at the option of
LESSOR  become  immediately  due and payable.  LESSOR  hereby  acknowledges  its
obligation  to  mitigate  its  damages  in the  event of a  default  by  LESSEE.
Notwithstanding the foregoing,  LESSEE agrees to pay reasonable  attorney's fees
incurred by LESSOR in  enforcing  any or all  obligations  of LESSEE  under this
Lease at any time.

         22.  NOTICE.  Any notice from  LESSOR to LESSEE  relating to the Leased
Premises or to the occupancy  thereof shall be deemed duly served if left at the
Leased  Premises  addressed  to  LESSEE,  or if sent to the Leased  Premises  by
certified mail, return receipt requested,  postage prepaid, addressed to LESSEE;
with a copy to  Testa,  Hurwitz  &  Thibeault,  LLP,  125 High  Street,  Boston,
Massachusetts  02110.  Any notice from  LESSEE to LESSOR  relating to the Leased
Premises or to the occupancy thereof shall be deemed duly served if delivered to
LESSOR by certified mail, return receipt requested,  postage prepaid,  addressed
to: The Abbey Corp.,  575 Boylston  Street,  Boston,  Massachusetts  02116 or at
LESSOR's last address  designated by written  notice to LESSEE.  Notice shall be
deemed  given at the earlier of the date of actual  delivery or if by  certified
mail,  three (3) business days after posting with the U.S. Postal Service.  Time
is of the  essence  in  delivery  of any  notice,  and  the  performance  of any
obligations relating thereto.












                                      -20-

         23.  OCCUPANCY.  In the event that LESSEE remains on the Premises after
the agreed  termination  date of this Lease  without the written  permission  of
LESSOR,  then all other terms of this Lease shall continue to apply, except that
LESSEE shall be liable to LESSOR for any loss,  damages or expenses  incurred by
LESSOR,  and all Annual  Base Rent and other  Rent shall be due in full  monthly
installments  at a rate of three  hundred  (300%)  percent of that  which  would
otherwise be due under this lease, it being understood  between the parties that
such extended  occupancy as a tenant at sufferance is solely for the benefit and
convenience of LESSEE.

         24. RULES AND  REGULATIONS.  LESSEE and LESSEE's  servants,  employees,
agents, invitees and licensees shall observe faithfully and comply strictly with
such reasonable and  non-discriminatory  rules and regulations governing the use
of the  Building  and site and all common areas as LESSOR may from time to time,
adopt,  provided  LESSEE  receives  reasonable  advance notice of such rules and
regulations  and such rules and  regulations  do not  conflict  with the express
provisions  of this Lease or  unreasonably  interfere  with  LESSEE's use of the
Leased Premises for its permitted purposes hereunder.

         25. OUTSIDE AREA. No goods or things of any type or  description  shall
be held or stored  outside the Leased  Premises at any time  without the express
written  approval of LESSOR,  except  bicycles which shall be stored only in the
bicycle rack to be provided by LESSOR.

         26. ENVIRONMENT. LESSEE will so conduct and operate the Leased Premises
as not to interfere in any way with the use and  enjoyment of other  portions of
the same or  neighboring  buildings  by others,  by reason of  offensive  odors,
smells,  noise,  accumulation  of  garbage  or trash,  vermin or other  pests or
otherwise and will, at its expense,  employ a professional  pest control service
if  necessary  as a result of  LESSEE's  operations.  LESSEE  agrees to maintain
efficient and effective device for preventing  damage to heating  equipment from
harmful  solvents,  degreasers,  cutting oils,  and the like,  which may be used
within the Leased  Premises.  No  hazardous  wastes,  radioactive  materials  or
chemical or harmful  biological  agents or materials of any sort shall be stored
or  allowed  to remain  within the Leased  Premises  at any time,  except  after
written  notice to LESSOR,  with  LESSEE  providing  a general  list of all such
substances  inclusive of approximate  quantities  and "MSDS"  sheets;  LESSEE to
comply  at all  times  with all  applicable  federal,  state,  and  local  laws,
ordinances,  regulations and administrative  orders governing the use, handling,
storage,  transportation and disposal of all such materials, as well as LESSOR's
rules and  regulations  governing  the  handling,  storage,  transportation  and
disposal  of all such  materials  within the  Building  and Common  Areas,  said
LESSOR's   rules  and   regulations  to  be   administered   in  a  uniform  and
non-discriminatory manner as to all tenants similarly situated.

         Prior to  vacating  the Leased  Premises at the end of the Term (or any
applicable extension), or sooner in the event of a default hereunder,  LESSEE at
its sole cost and expense shall provide  LESSOR and Owner with an  environmental
audit by a qualified







                                      -21-

environmental   engineering   firm   satisfactory   to  LESSOR.   The  aforesaid
environmental  audits shall duly  recognize the substances and materials used by
the LESSEE in its laboratory and office operations on the Leased Premises during
the  Lease  Term  (the  "Known  Substances"),  and  shall  confirm  to  LESSOR's
reasonable  satisfaction  that the  Leased  Premises  are free  from any and all
contaminants,  pollutants, radioactive materials, hazardous wastes or materials,
bacteriological  agents or organisms  which would render the Leased  Premises in
violation of G.L. c. 21E,  CERCLA,  or SARA, or any  regulations  of the NRC, as
they may be amended, or supplemented by administrative regulations, from time to
time;  arising  from  and  based  on  the  Known  Substances.  LESSEE  shall  be
responsible  to  LESSOR  (and  any  Lenders  to the  Building)  for  any and all
environmental  hazards or conditions  which  preclude or condition the foregoing
confirmation  due from LESSEE as contemplated  above, to the extent said hazards
or conditions are reasonably  attributable to the Known  Substances and LESSEE's
activities and use of their space.

         27. RESPONSIBILITY.  LESSOR shall not be held liable to anyone for loss
or  damage  caused  in any way by the use,  leakage  or  escape  of water or for
cessation  of any  service  rendered  customarily  to said  Leased  Premises  or
buildings or agreed to by the terms of this Lease,  due to any accident,  to the
making of repairs, alterations or improvements,  to labor difficulties,  weather
conditions,  or mechanical breakdowns, to trouble or scarcity in obtaining fuel,
electricity,  service or supplies  from the sources  from which they are usually
obtained  for said  building,  or to any cause  beyond the  LESSOR's  reasonable
immediate  control,  except to the extent  that such loss or damage is caused by
the  negligence  or willful  misconduct of LESSOR or its agents,  employees,  or
independent contractors.

         28.  SURRENDER.  LESSEE shall at the expiration or other termination of
this Lease remove all of LESSEE's personal property,  goods and effects from the
Leased  Premises.  LESSEE  shall  deliver to LESSOR the Leased  Premises and all
keys,  locks,  thereto,  and other  fixtures and equipment  connected  therewith
(excluding LESSEE's trade fixtures and equipment) and all alterations, additions
and improvements made to or upon the Leased Premises,  including but not limited
to any plumbing and plumbing fixtures,  air conditioning  equipment and ductwork
of any type, exhaust fans or heaters, water coolers,  burglar alarms,  telephone
wiring,  telephone  equipment,  wooden or metal  shelving which has been bolted,
welded or otherwise  attached to any  concrete or steel member of the  Building,
compressors,  air or gas distribution piping, and all electrical work, including
but not  limited  to  lighting  fixtures  of any  type,  wiring,  conduit,  EMT,
distribution panels, bus ducts, raceways,  outlets and disconnects to the extent
that LESSOR  constructs or installs such  fixtures,  alterations,  additions and
improvements at LESSOR's expense. Notwithstanding any contrary provision of this
Lease,  at the end of the Lease Term, and absent any default,  LESSEE shall have
the right to remove from the Leased  Premises  its trade  fixtures  and personal
property and any fixtures, alterations,  additions and improvements installed by
LESSEE or at LESSEE's  cost and expense  subsequent  to the  Commencement  Date,
including  without  limitation  the fixtures and  equipment  listed on Exhibit F
hereto,  provided  that LESSEE  shall  repair any damage to the Leased  Premises
caused thereby. LESSEE shall deliver the Leased Premises in as good condition as








                                      -22-

existed on the Commencement  Date,  reasonable wear and tear and damage by fire,
eminent  domain,  or other  casualty  only  excepted.  In the event of  LESSEE's
failure to remove any of LESSEE's  property from the premises,  LESSOR is hereby
authorized,  without  liability to LESSEE for loss or damages thereto and at the
sole risk of LESSEE to remove and store any such  property at LESSEE's  expense,
or to retain same under  LESSOR's  control or to sell at public or private sale,
any or all of the  property not so removed and to apply the net proceeds of such
sale to the payment of any sum due hereunder,  or to destroy such property which
shall be conclusively deemed to have been abandoned.

         29. QUIET  ENJOYMENT.  So long as LESSEE  keeps,  observes and performs
each of the terms herein  contained  on the part of LESSEE to be kept,  observed
and performed,  LESSEE shall quietly enjoy the Leased Premises without hindrance
or  molestation  by LESSOR,  or any party  claiming any interest in the Building
under or through the LESSOR.

         30. LESSOR'S SERVICES.  LESSOR will provide elevator facilities serving
the Leased Premises at all times, subject to reasonable security regulations for
use at times  other than  Normal  Business  Hours.  LESSOR  shall,  at  LESSOR's
expense,  furnish  LESSEE such  cleaning  service as is  described  in Exhibit D
hereto.  Except as set forth on Exhibit D, LESSOR shall not be  responsible  for
cleaning LESSEE's research laboratories.

         31.  GENERAL.  The invalidity or  unenforceability  of any provision of
this  Lease  shall not  affect  or render  invalid  or  unenforceable  any other
provision  hereof.  The  obligations  of this Lease shall run with the land, and
this Lease shall be binding upon and inure to the benefit of the parties  hereto
and their respective successors and assigns,  except that LESSOR shall be liable
only for  obligations  during  LESSOR's  ownership of the Building or Landlord's
interest  under this Lease,  occurring  as of the  beginning of the term of this
Lease,  or thereafter  while LESSOR of the entire  Property.  The obligations of
LESSOR shall not be binding upon any director,  officer,  shareholder,  partner,
trustee or  beneficiary  of LESSOR.  Notwithstanding  the  definition  herein of
"Commencement  Date," "Termination Date," or "Term," or LESSOR's  obligations to
deliver the Premises, this Lease shall be binding and enforceable as against the
parties hereto as of the date of its execution.

         32. WAIVERS,  ETC. No consent or waiver,  express or implied, by LESSOR
or  LESSEE,  to or of any  other  breach  of the  other  party of any  covenant,
condition  or duty of that party shall be construed as a consent or waiver to or
of any other breach of the same or any other  covenant,  condition  or duty.  If
LESSEE is several  persons or a partnership,  LESSEE's  obligations are joint or
partnership  and also  several.  Unless  repugnant to the context,  "LESSOR" and
"LESSEE" mean the person or persons, natural or corporate, named above as LESSOR
and  as  LESSEE   respectively,   and   their   respective   heirs,   executors,
administrators, successors and assigns.











                                      -23-

         33. LESSOR'S OBLIGATION TO "BUILD OUT" AND DELIVER LEASED PREMISES. The
Leased   Premises   shall  be  delivered  in  an  "as  is"   condition   without
representation  or warranty as to the  suitability  of the Leased  Premises  for
LESSEE's  particular  uses  and  activities  (but  for the  representations  and
warranties set forth in paragraph 8 hereof); except that the LESSOR, at LESSOR's
expense,  shall:  Repaint the  Premises;  repair/replace  carpeting as and where
necessary  based on its current wear to put the carpeting in the Leased Premises
in good  condition;  prepare  LESSOR's  basic and  standard  infrastructure  for
LESSEE's  laboratory space; and provide lighting and electrical  distribution in
the Premises in accordance with the building standards;  as further set forth in
paragraph  11 hereof;  and  otherwise  construct  and install  the  alterations,
improvements, fixtures and equipment shown and or described on Exhibit C (in its
component parts,  inclusive of "Addendum to Exhibit C"),  attached to this Lease
(collectively,  the "LESSOR's Work"). Any specialized  construction  required to
satisfy LESSEE's laboratory needs shall be LESSEE's sole responsibility (and may
be provided by LESSOR based on separate agreement as to cost and timing),  (See,
separate schedule of additional costs dated June 18, 1997 and attached at end of
Exhibit C).

         LESSOR  shall  perform its  LESSOR's  Work on the  Premises,  and shall
deliver said Premises  subsequent to completion of LESSOR's Work, in two phases:
"Phase  I" shall  consist  of  LESSOR's  Work on the  portions  of the  Premises
dedicated  to office and general use (i.e.,  the "Office  Space"  consisting  of
approximately  9,759  square  feet as shown on  Exhibit A - 1  attached  to this
Lease) and other portions of the Building  common areas needed for LESSEE to use
and occupy the Office Space; and "Phase II" shall consist of LESSOR's Work other
than as  included  in Phase I (i.e.,  approximately  2,503  square  feet  mostly
dedicated to laboratory  use). Upon  substantial  completion of LESSOR's Work in
Phase I, it shall  notify  LESSEE in  writing,  in the form  attached  hereto as
Exhibit E - 1, of the  completion of said LESSOR's Work and the delivery of that
portion of the Premises to LESSEE.  The date of  completion  of LESSOR's Work as
stated on said notice to LESSEE is  referred to as the "Phase I Delivery  Date."
Upon substantial completion of LESSOR's Work in Phase II, it shall notify LESSEE
in writing,  in the form attached  hereto as Exhibit E - 2, of the completion of
said  LESSOR's  Work and the delivery of that portion of the Premises to LESSEE.
The date of  completion  of LESSOR's  Work as stated on said notice to LESSEE is
referred to as the "Phase II Delivery Date."

         LESSOR  shall  deliver  its  Phase I  Delivery  Date  notice  upon  its
completion  of LESSOR's  Work on Phase I, on or about  September  1, 1997 but no
later than  September  15, 1997;  and LESSOR shall deliver its Phase II Delivery
Date notice upon  LESSOR's  completion  of LESSOR's Work on Phase II on or about
October 1, 1997,  but no later than November 1, 1997.  LESSOR's  obligations  to
complete  as  stated  above  shall  be  extended  on a day to day  basis  by its
inability  to complete  due to  governmental  restrictions;  strikes;  lockouts;
shortage  or  labor  or  materials;   civil   commotion;   extreme   weather  or
environmental conditions; or acts of God.










                                      -24-

         LESSOR shall complete  LESSOR's Work in a good and workmanlike  manner,
according  to the plans and  specifications  attached  hereto as  Exhibit  C, in
compliance with applicable laws,  codes,  ordinances and  regulations.  The term
"substantial completion" as used herein shall mean the completion of all work in
that  respective  Phase I or Phase II as the case may be, in accordance with the
plans and  specifications;  but for commonly accepted  "punchlist  items," which
shall be in the process of being completed (or which need to be completed at the
end of all the Work) and which do not materially interfere with LESSEE's ability
to use the space being delivered for its intended purpose (e.g. office space for
office use;  laboratory  space for  laboratory  use).  LESSOR shall complete its
punchlist  work within thirty (30) days after the Phase II Delivery  Date.  Upon
completion of LESSOR's Phase I Work and by its delivery  notice  attached hereto
as  Exhibit E - 1.  LESSOR  shall  have  certified  that  portion  of the Leased
Premises is valid for occupancy as office space; and subsequent to completion of
LESSOR's  Phase II Work, it shall  deliver to LESSEE a certificate  of occupancy
for the laboratory space.

         If any  change in the scope or timing of  LESSOR's  Phase I or Phase II
Work is necessitated by any changes in the plans and specifications for LESSOR's
work  requested  by  LESSEE,  or any  additions  thereto,  then  LESSOR  in each
instance, as soon as practicable under the circumstances,  obtain an estimate of
the increase in the cost of such change and/or any  anticipated  delay resulting
therefrom.  LESSEE must decide whether to proceed with such requested  change as
soon as practicable under the  circumstances  after receipt of LESSOR's estimate
of cost and/or delay (but if  occasioned  by a change  requested by LESSEE after
commencement of the actual  construction of the Phase I or Phase II Work, within
forty-eight (48) hours). LESSOR's statement shall set forth the additional costs
for any changes  that result in  additional  work,  and the credits  against the
original  cost of the work for any  changes  that  result in  deletion  from the
original  scope of work.  If LESSEE  decides to  proceed,  then it shall  notify
LESSOR in writing within said timeframes. Upon an affirmative decision by LESSEE
to proceed,  the cost of the change shall be paid to LESSOR  immediately  and in
advance  and the  work  shall  be  performed  as part of the  applicable  Phase.
Additionally,  if LESSEE decides to proceed,  any delay occasioned by the change
shall extend, day for day, LESSOR's delivery obligations and deadlines hereunder
accordingly (but shall not change the date on which the Interim Rent or the full
Annual Base Rent would have commenced under LESSOR's  construction  schedule but
for the change invoked by LESSEE;  and each of those  respective  Rents shall be
paid to LESSOR as of the dates they would  have  become due absent the  LESSEE's
changes,  notwithstanding  the  provisions of the second  paragraph of section 2
hereof to the contrary).

         LESSEE shall pay its pro rata share of Annual Base Rent and  Additional
Rent for any  portion  of the  initial  calendar  month in  which  LESSOR  makes
delivery of the Leased  Premises,  on  delivery,  as  contemplated  in section 2
hereof.

         34.  OPTION TO EXTEND.  LESSEE,  provided  it is not then in default in
each instance  hereunder beyond any applicable cure periods shall have an option
to extend  the Lease  Term,  on the terms  and  conditions  herein,  for one (1)
additional  period of five 








                                      -25-

(5)  years,  with the  Annual  Base Rent to be the then  current  "Market  Rent"
(including annual escalations  thereon for each year of the extended term, based
on increases in the Consumer Price Index or fixed  increases,  as he case may be
as determined by then prevailing market forces),  (herein, the "Extended Term").
Said Extended Term shall  commence,  subject to proper exercise in each instance
of LESSEE's option hereunder,  on the Termination Date of the original Term, and
shall  terminate on that date which is the fifth  anniversary of the Termination
Date.  For  example,  should the  Termination  Date be September  30, 2002,  the
Extended Term terminates on September 30, 2007.

         LESSEE shall  exercise its option by  delivering  to LESSOR its written
notice not later than August 30, 2001.  "Market  Rent" (which term shall include
annual  escalations) shall be that fair market rent charged for comparable first
class  research  laboratory  and office  space in Cambridge as of the end of the
original  Term.  If, after good faith attempts for thirty (30) days after LESSEE
delivers  notice to  LESSOR,  the LESSOR  and  LESSEE  cannot  agree on a figure
representing  Market Rent, then either party,  upon written notice to the other,
may  request  arbitration  of the  issue as  provided  in this  section.  Within
fourteen  (14) days of the request for  arbitration,  each party shall submit to
the other the name of one unrelated  individual or entity with proven  expertise
in the  leasing of  commercial  real  estate in greater  Boston to serve as that
party's  appraiser.  Each appraiser  shall be paid by the party selecting him or
it. The two appraisers  shall meet within  fourteen (14) days of their selection
and   collaboratively   determine  the  Market  Rent.   They  shall  make  their
determination  in writing,  including a statement if such is the case, that they
are at an impasse. Such a statement of impasse shall be submitted to the parties
along with the Market Rent figure  which each  appraiser  has  selected  and his
reasons and  substantiation  therefor.  The  appraisers,  in case of an impasse,
shall  also  agree on one  unrelated  individual  or entity  with  expertise  in
commercial  real  estate  in  greater  Boston  with at  least  seven  (7)  years
experience dealing with real estate appraisals who shall evaluate the reports of
the two original  appraisers and within  fourteen (14) days of submission of the
issue to him,  make his own  determination  as to a figure  representing  Market
Rent. The determination of this individual or entity (i.e.,  arbitrator) absent,
fraud,  bias or undue prejudice  shall be binding upon the parties.  In no event
shall "Market  Rent" for the Extended  Term be less than that figure  payable by
LESSEE  during  the last  year of the Lease  Term  increased  by the  cumulative
increase in the greater Boston Consumer Price Index All Urban, all items,  since
the last increase in Annual Base Rent.

         Annual Base Rent and Additional  Rent during any Extended Term shall be
payable  in  advance,  in equal  monthly  installments  on the first day of each
calendar month.

         35.  EXTENDED  TERM  ADDITIONAL  RENT.  LESSEE in  addition to the sums
payable  annually  to LESSOR as Annual  Base Rent,  shall pay to LESSOR for each
year of any Extended Term, as Additional  Operating Expense Rent, and Additional
Tax  Escalation  Rent,  LESSEE's  Allocable  Percentage  (as  determined  by the
approximate total rentable space leased),  of any and all increases in operating
expenses  and taxes,  as  contemplated  in Section 3 and 4 of this  Lease,  over
Exhibit B attached hereto.











                                      -26-

         36 ESTOPPEL CERTIFICATE.  Upon not less than fifteen days prior written
request by LESSOR,  LESSEE shall  execute,  acknowledge  and deliver to LESSOR a
statement in writing  certifying  if true that this Lease is  unmodified  and in
full  force and  effect and that  LESSEE  has at the time of such  statement  no
defenses,  offsets or  counterclaims  against its obligations to pay Annual Base
Rent and Additional  Operating  Expense Rent and Additional Tax Escalation  Rent
and any other charges and to perform its other  covenants  under this Lease (or,
if there have been any  modifications  that the same is in full force and effect
as  modified  and  stating the  modifications  and,  if there are any  defenses,
offsets or  counterclaims,  setting them forth in  reasonable  detail),  and the
dates to which the Annual Base Rent and any  Additional  Rent and other  charges
have been paid.  Any such  statement  delivered  pursuant to this Section may be
relied upon by any  prospective  purchaser or mortgagee of the Premises,  or any
prospective assignee of any such mortgagee or the LESSOR.

         37. PRIOR NOTICE AS TO ADJACENT SPACE. LESSEE,  provided it is not then
in default  hereunder,  beyond any applicable notice and cure periods,  shall be
provided at least fifteen (15) days written notice prior to LESSOR's remarketing
of the space  adjacent to the Leased  Premises  hereunder on the fourth floor of
the Building;  subject to any existing  exclusive rights to said space conferred
on any other  lessees in the  Building.  During  said  fifteen  (15) day period,
LESSOR shall entertain offers from and negotiate in good faith with LESSEE as to
the market terms and  conditions of any  prospective  occupancy of said adjacent
space by LESSEE.  Once said fifteen (15) day period has lapsed,  LESSOR shall be
free to actively  market and negotiate with any party and, if  discussions  were
initiated by LESSEE  within said fifteen (15) day period,  shall also  negotiate
with  LESSEE in good  faith as to said  space.  Notwithstanding  the  foregoing,
LESSOR  shall not be bound to  consummate  any  transaction  for said space with
LESSEE.

         38. LESSEE'S LENDER.  LESSEE intends to procure financing which will be
secured by various  articles  of  personal  property,  including  inventory  and
equipment  to  be  located  in  and  upon  the  Leased  Premises  (the  "Secured
Personalty"),  which Secured  Personalty is to be covered by or described in one
or more security agreements made and entered into by LESSEE.  LESSOR agrees that
none of the aforesaid collateral which is the subject of any security agreements
granting a security  interest to any of such lenders shall be subject to a claim
for Rent or liable for any lien,  right or claim of LESSOR that is senior to the
claim of the lender.  Furthermore,  in the event the lender,  its  successors or
assigns  shall  at any time  exercise  its or their  right or power  under  said
security  agreements to take possession of or to remove the Secured  Personalty;
and if  LESSOR  receives  at least  ten (10) days  prior  written  notice of the
exercise  of said rights with  respect to any of the Secured  Personalty,  other
than LESSEE's books and records (for which no advance notice shall be required),
said notice to provide the name,  address,  phone number and contact  persons in
charge of the exercise of said lender's rights;  LESSOR will not hinder the same
or object  thereto or interfere  therewith,  and LESSOR hereby  consents to such
taking of possession and removal of said Secured  Personalty,  regardless of the
manner or degree of their attachment to the realty of the improvements  thereon,
and consents to the entry upon said Leased Premises for such purposes;  provided
that the secured lender shall 









                                      -27-

observe  LESSOR's  reasonable  rules and  regulations  governing  the  manner of
removal of the Secured Personalty from the Leased Premises and the building, and
further provided the secured lender will repair any and all damage occasioned by
said  removal,   properly   capping  all  pipes  and  repairing  all  holes  and
protrusions,  and will pay LESSOR Rent at the daily rate set forth in this Lease
for the Leased  Premises if it occupies the Leased  Premises for longer than one
(1) day. At LESSEE's request, LESSOR shall execute an instrument in favor of the
secured lender confirming the terms of this section.

         39.  GOVERNING  LAW.  This  Lease  constitutes  the full  and  complete
agreement  between  the  LESSOR  and  LESSEE  and shall be  construed  under and
according to the laws of the  Commonwealth  of Mass. Any provision of this Lease
which is deemed void or  unenforceable  shall not  invalidate  or render void or
unenforceable the entire Lease. This Lease may only be modified by instrument in
writing  signed by both  parties  hereto  (or  their  permitted  successors  and
assigns).

         IN WITNESS WHEREOF, LESSOR AND LESSEE have hereunto set their hands and
seals this 3rd day of July, 1997.

                                     LESSOR:

                                     RIVERTECH ASSOCIATES, LLC


                                     By:  /s/ Robert Epstein
                                          --------------------------------------
                                          Robert Epstein, Member/Manager

LESSEE:

DYNAGEN, INC.


By:  /s/ Indu A. Muni
   --------------------------
its duly authorized President