EXHIBIT 10.6

                               SUBLEASE AGREEMENT

This Sublease Agreement is made as of May 1, 1996 and is by and between TCell
Sciences, Inc., a Delaware corporation with a principal address at 115-119
Fourth Avenue, Needham, Massachusetts ("Sublessor"), and Repligen Corporation, a
Delaware corporation with a principal address at 117 Fourth Avenue, Needham,
Massachusetts ("Sublessee").

Reference is made to the following facts:

A. As of May 1, 1996, Fourth Avenue Ventures Limited Partnership (the
"Landlord") entered into a lease with Sublessor (the "Overlease") a true and
correct copy of which is attached hereto as Exhibit A, for certain office and
laboratory space (the "Main Premises") within the building located at 115-119
Fourth Avenue, Needham, Massachusetts, as more specifically described in the
Overlease (the "Building")

B. Subject to the terms and conditions of this Sublease Agreement, Sublessor has
agreed to sublease to Sublessee a portion of the Premises hereinafter referred
to as the "Subleased Premises." The Subleased Premises shall consist of certain
space to be used exclusively by Sublessee, which space is shown on Exhibit B
attached hereto and incorporated herein. For the purposes of determining
Sublessee's Annual Fixed Rent and Sublessee's proportionate share of taxes,
utilities and operating expenses, the square footage of the Subleased Premises
is approximately 13,000 rentable square feet.

C. Capitalized terms not otherwise defined herein shall have the meaning set
forth in the Overlease.

AGREEMENTS:

In consideration of the mutual covenants herein contained and for other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, Sublessor and Sublessee hereby agree as follows:

1. Premises. Sublessor hereby leases to Sublessee, and Subleases hereby leases
from Sublessor, the Subleased Premises for all Permitted Uses described in the
Overlease, provided that such use is permitted by law or pursuant to a special
permit duly issued by the Town of Needham and in full force and effect. The
Subleased Premises consist of approximately 13,000 rentable square feet as shown
on Exhibit B attached hereto and incorporated herein by this reference.
Sublessee shall have, as appurtenant to the Subleased Premises, rights to use,
in common with others entitled thereto: (i) common lobbies, hallways, ramps,
stairways, freight elevators, shipping/receiving areas, and other common areas
of the Building, if any; (ii) the common walkways and driveways necessary for
access to the Building; (iii) the common hallways, emergency exits, stairways
and elevators within the Main Premises necessary for access to the Subleased
Premises, all as shown on Exhibit B, (iv) the HVAC systems and common utility
equipment, pipes, ducts, conduits, wires and appurtenant equipment serving the
Subleased Premises and (v) on a nonexclusive basis, twenty five (25) parking
spaces.



a. Sublessor shall deliver the Subleased Premises to Sublessee on May 1, 1996
(the "Commencement Date") in such condition as exists as of the date of this
Sublease, free of all occupants other than Sublessee.

2. Term. To have and to hold for a term of four (4) years, commencing on May 1,
1996 and terminating on April 30, 2000 unless sooner terminated as herein
provided (the "Term").

Provided that Sublessor has not exercised its option to terminate the Overlease
on April 30, 2000, as provided in the Overlease, Sublessee shall have the right
to extend the term of this Sublease for two years, such that the Sublease will
expire on April 30, 2002. In order to exercise this option, Sublessee must
provide Sublessor with written notice of its exercise no later than nine (9)
months prior to the expiration of this Sublease.

Provided that Sublessee has exercised its extension option and is not then in
default hereunder beyond the expiration of any applicable cure period, Sublessee
shall have the right to terminate this Sublease at any time after May 1, 2000
upon the following terms and conditions: Sublessee must deliver to Sublessor a
notice of its intent to terminate this Sublease at least nine (9) months prior
to the proposed termination date. Such notice of termination may be given no
earlier than May 1, 2000, and, to be effective, such notice of termination must
be accompanied by a payment of $100,000.00 in good and immediately available
funds, and upon Sublessor's receipt of such notice and the $100,000.00 payment,
the termination date of this Sublease shall be changed to the date that is nine
(9) months after the date of such notice.

Effective on May 1, 2001, the termination payment for any notice of termination
given on or after May 1, 2001, will be reduced to $50,000.00, provided that the
Sublessee otherwise complies with all provisions of the previous paragraph.

3. Fixed Rent. Sublessee covenants and agrees to pay to Sublessor without setoff
or deduction, rent (the "Fixed Rent") in equal monthly installments in advance
on the first day of each month of the Term at Sublessor's address shown herein
or at such place as Sublessor shall from time to time designate in writing as
follows:

             Lease Year             Monthly Fixed Rent
             ----------             ------------------
      May 1, 1996-June 30, 1997        $ 9,208.33
      July 1, 1997-April 30, 2000      $11,136.66

a. If the Sublessee extends the Term as provided in Section 2 hereof, from May
1, 2000 and continuing to April 30, 2002, Sublessee will pay monthly rent of
Eleven Thousand One Hundred Thirty Six Dollars and 66/100 ($11,136.66).

b. If the Term includes a partial calendar month at its beginning or end, the
monthly installment of Rent for such partial month shall be prorated at the rate
of 1/30 of the monthly installment for each day in such partial month within the
Term and shall be payable in advance on the first day of such partial month
occurring within the Term.



4. Additional Rent. Sublessee covenants and agrees to pay monthly to Sublessor
as additional rent, upon payment of monthly installments of Fixed Rent,
Sublessee's proportionate share of all Operating Expenses and utilities due from
Sublessor to Landlord or any public utility under Sections 5 and 7 of the
Overlease on account of the Subleased Premises. Sublessee's proportionate share
is defined herein as 24%.

5. Additional Sublessor Covenants. Sublessor hereby covenants:

a. To pay all rents as they come due; subject to the provisions of Section 6(b)
below, to repair and maintain in good working order the Subleased Premises,
including the HVAC and utility systems exclusively serving the Main Premises and
the Subleased Premises and to timely perform all of its other obligations under
the Overlease; and to promptly provide Sublessee with copies of all notices it
receives under the Overlease which relate to the Subleased Premises.

b. Sublessor further agrees to indemnify and hold Sublessee harmless from any
and all damages, costs, losses and the like suffered by Sublessee as a result of
a breach by Sublessor of any of its obligations arising under or pursuant to the
Overlease, said indemnification to include, without limitation, reasonable
attorney's fees, unless such breach was caused by a default of Sublessee
hereunder.

c. That Sublessee, upon paying the rent and performing the covenants of this
Sublease Agreement, shall peacefully and quietly have, hold and enjoy the
Subleased Premises and all rights granted to Sublessee in this Sublease
Agreement throughout the Term.

d. To diligently pursue enforcement of Landlord's obligations under the
Overlease if failure to perform such obligations materially, adversely affects
Sublessee's use or occupancy of the Subleased Premises. In no event shall
Sublessee have an independent right to enforce the Landlord's obligations under
the Overlease, nor shall Sublessee be subrogated to the rights of Sublessor to
enforce the Landlord's obligations under the Overlease.

e. For so long as Sublessee is not in default hereunder beyond applicable cure
periods, to provide Sublessee with support services, in addition to those
provided to the Main Premises by Landlord pursuant to the provisions of the
Overlease, for Sublessee's operations in the Subleased Premises. Sublessee shall
pay Sublessee's proportionate share of the cost for such support services as
additional rent upon payment of installments of Fixed Rent. Sublessee and
Sublessor will negotiate a mutually satisfactory services agreement for the
provision of such services to Sublessee by Sublessor.

f. To obtain and maintain all facility-based (as opposed to use or operator
based) local, state and federal permits and approvals necessary to operate the
Main Premises and the Subleased Premises for all Permitted Uses.

g. To indemnify, defend and hold Sublessee harmless from and against any claim
arising from Sublessor's use of the Main Premises; or the conduct of its
business or from any activity, work or thing done, permitted or suffered by
Sublessor on or about the Main Premises, or arising from any breach of
Sublessor's obligations hereunder, and from any claim for injury or damage to
any person or property while on or about the Sublessor's Premises; except for
any claim arising solely as a result of the negligence or willful misconduct of
Sublessee or Sublessee's employees or agents. This indemnification



shall specifically include indemnification under the provisions of Section 13.2
of the Overlease with respect to any Hazardous Substance on the Subleased
Premises.

h. To promptly report to Sublessee any incidents in which its employees, agents
or contractors report an illness or other adverse health effects attributable to
the Subleased Premises, the Building or any of the equipment, fixtures or
furnishings therein.

6. Additional Sublessee Covenants. Sublessee hereby covenants:

a. To comply with all the terms and provisions of the Overlease applicable to
the Subleased Premises except those terms that apply to the payment of Fixed
Rent and additional rent, and except Sections 9.15, 15.2, and 15.3 of the
Overlease, and to do nothing which will subject the Overlease to termination by
Landlord under the provisions of the Overlease.

b. To maintain the Subleased Premises in the same condition as they are at the
commencement of the Term, reasonable wear and tear, damage by fire and other
casualty and eminent domain excepted. In the event that Sublessee defaults in
its obligations under this paragraph (b), Sublessor may, after at least ten (10)
days prior notice to Sublessee (except in an emergency, when no notice will be
required) cure such default, in which event Sublessee shall reimburse Sublessor
for any costs incurred by Sublessor to cure such default.

c. To remove all of Sublessee's personal property, business equipment, goods and
effects from the Subleased Premises at the earlier expiration or termination of
this Sublease Agreement and to deliver to Sublessor the Subleased Premises in
the same condition as they were at the commencement of the Term, or as they were
put in during the Term, reasonable wear and tear, damage by fire or other
casualty or eminent domain excepted. In the event of Sublessee's failure to
remove any of Sublessee's property from the Subleased Premises, Sublessor is
hereby authorized, without liability to Sublessee for loss or damage thereto,
and at the sole risk of Sublessee to remove and store any of such property at
Sublessee's expense, or to retain the same under Sublessor's control or to sell
at public or private sale, upon at least ten (10) days notice, any or all of the
property not so removed and to apply the net proceeds of such sale to the
payment of any sums due hereunder, or to destroy such property. Any remaining
balance will be paid to Sublessee.

d. In addition to the indemnification and obligations set forth in Section 13.2
of the Overlease (which apply to the Subleased Premises), to indemnity, defend
and hold the Sublessor harmless from and against any claim arising from
Sublessee's use of the Subleased Premises or the conduct of its business or from
any activity, work or thing done, permitted or suffered by Sublessee on or about
the Subleased Premises, or arising from any breach of Sublessee's obligations
hereunder, and from any claim for injury or damage to any person or property
while on or about the Subleased Premises, except for any claim arising solely as
a result of the negligence or willful misconduct of Sublessor or Sublessor's
employees or agents. This indemnification shall specifically include
indemnification under the provisions of Section 13.2 of the Overlease with
respect to any Hazardous Substance on the Subleased Premises.

e. To maintain comprehensive general liability insurance in amounts which shall,
at the beginning of the Term, be not less than those limits set forth in Section
6 of the Overlease, and from time to time during the Term, in such additional
amounts which are customarily carried in the Greater Boston area on



property similar to the Building and used for similar purposes; Sublessee shall
also maintain all-risk property insurance for Sublessee's property. Prior to the
commencement of the Term, Sublessee shall deliver to Sublessor certificates of
all required insurance coverage naming Sublessor, Landlord and Landlord's
Mortgagee as additional insureds.

f. To obtain and maintain all local, state and federal permits and approvals
necessary solely for the conduct of Sublessee's business in the Subleased
Premises.

g. To comply with all applicable local, state and federal laws and cooperate
with Sublessor's health and safety regulations applicable to the Subleased
Premises.

h. To use the Subleased Premises only for the Permitted Uses set forth in the
Overlease, in compliance with applicable law, provided that if Sublessee's
production of any biological products shall result in a material increase in the
utility usage of the Subleased Premises, or if Sublessor's production of any
biological products shall result in a material increase in the utility usage of
the Main Premises, the parties hereto shall mutually agree upon an adjustment in
Sublessee's proportionate share of the cost of utilities to be paid by Sublessee
to Sublessor in accordance with the provisions of Paragraph 4 hereof. Subject to
the foregoing, Sublessor and Sublessee also agree that Sublessee shall have the
right to conduct animal research in the Subleased Premises, provided that (1)
Sublessee obtains all necessary federal, state and local approvals for such use
prior to commencement of such use, and (2) Sublessee and Sublessor mutually
agree upon the location of the room(s) in which the animals will be housed,
Sublessee's procedures for maintaining and operating such room and the animals
contained in such room. Sublessor shall thereafter have the right to monitor
Sublessee's compliance with all necessary permits for maintaining such use.
Sublessor's consent to the location of such animal room and Sublessee's
procedures for maintaining and operating such room will not be unreasonably
withheld, conditioned or delayed.

i. To appoint a representative or representatives to serve on the relevant
health and safety committees for the Building, and to provide Sublessor with
copies of all safety committee minutes, and to promptly notify Sublessor of any
safety concerns of Sublessee that arise out of such committee meetings, or
otherwise.

j. To regularly perform janitorial services and extermination services, as
necessary, to the Subleased Premises.

k. To, in accordance with a separate agreement with Landlord, construct
$250,000.00 of improvements to the Subleased Premises, which improvements will
remain part of the Subleased Premises on the expiration of the Term, and may not
be removed by Sublessee, unless such removal is required by Sublessor at the
time Sublessor approves the construction of such improvements. Sublessor shall
have the right to consent to the construction of such improvements, and in
obtaining such consent Sublessor and Sublessee shall follow the procedures set
forth in Section 10.b of a certain Settlement, Escrow and Security Agreement of
even date between Sublessee and Landlord, and Section 10.b of such Agreement is
incorporated herein by reference. Sublessor has consented to the schematic
showing some of the planned improvements that is attached hereto as Exhibit C,
and will not unreasonably withhold its consent to the remainder of the
improvements planned by Sublessee.



l. To be responsible for compliance with the provisions of the Americans with
Disabilities Act with respect to the Subleased Premises and all improvements to
be constructed in the Subleased Premises.

7. Condition and Alterations. By its execution hereto, Sublessee acknowledges
that:

a. Sublessee has inspected the Subleased Premises and accepts the same "AS IS."
Sublessor shall have no obligation or duty to Sublessee regarding the
preparation of the Subleased Premises for occupancy of Sublessee.

b. Other than the improvements to be constructed by Sublessee in accordance with
Section 6(k) hereof, in the event Sublessee desires to make alterations or
improvements to the Subleased Premises, Sublessee shall obtain the prior written
consent of both Sublessor and Landlord prior to conducting any such alterations
or improvements, and the approval process shall be as outlined in Section 9.4 of
the Overlease.

8. Fire, Casualty and Eminent Domain. In the event the Subleased Premises, or a
portion thereof, are rendered substantially unsuitable for their intended use by
fire or other casualty or are taken by eminent domain, a just and proportionate
abatement of rent shall be made to the extent Sublessor is entitled to an
abatement of rent under the Overlease. Sublessee hereby releases and assigns to
Sublessor all of Sublessee's right, title and interest in and to any and all
damages or injury to the Subleased Premises for any taking by eminent domain or
by reason of anything lawfully done in pursuance of public authority. Sublessee
further covenants to deliver such further assignment and assurances thereof as
Sublessor may from time to time request, hereby irrevocably designating and
appointing Sublessor as its attorney-in-fact to execute anal deliver in
Sublessee's name and behalf all such further assignments thereof.

9. Common Space. Sublessor and Sublessee shall share use of the common areas
marked on Exhibit B, the common loading and receiving areas, and the common
hallways. Sublessor shall maintain such areas, as provided in the Overlease, and
Sublessee shall pay Sublessor for such maintenance as provided in the services
agreement to be signed by Sublessor and Sublessee.

10. Access to Subleased Premises. Sublessee shall, promptly after execution of
this Sublease, and no later than ninety (90) days after execution of this
Sublease, install at Sublessee's sole cost, a new front entry for the Subleased
Premises in the location shown on Exhibit B, and to construct all walls
necessary to physically separate the Subleased Premises from the Main Premises.
Such entry and walls shall be constructed to the satisfaction of both Sublessor
and Landlord. In addition, Sublessee shall have access to the Main Premises to
remove its equipment from the Main Premises, as provided in a Settlement, Escrow
and Security Agreement between Landlord and Sublessee of even date, such access
to terminate on June 15, 1996; or to construct the improvements to be
constructed in accordance with Section 6(k) hereof, if such access is necessary.
All such access shall be upon prior notice to Sublessor, shall be at Sublessee's
sole risk, notwithstanding any provision to the contrary herein, shall not
unreasonably interfere with the Sublessor's operation of its business, and
Sublessee indemnifies Sublessor from all claims, loss or damage related to such
access, and Sublessee agrees not to store any equipment in any conference room
of Sublessor or other location, during any such period of access.

11. Estoppel Certificate. Sublessee agrees, from time to time, upon not less
than ten (10) days' prior written request by either Sublessor or Landlord, to
execute, acknowledge and deliver to Sublessor a



statement in writing, addressed to such party as either Sublessor or Landlord
shall designate in its notice to Sublessee, certifying that this Sublease
Agreement is unmodified and in full force and effect and that Sublessee has no
defenses, offsets or counterclaims against its obligations to pay the Fixed Rent
and additional rent and to perform its other covenants under this Sublease
Agreement (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, counterclaims or defaults, setting them forth in reasonable
detail), the dates to which the Fixed Rent and additional rent have been paid, a
statement that Sublessor is not in default hereunder (or if in default, the
nature of such default, in reasonable detail and any additional information
reasonably requested by either Sublessor or Landlord. Any such statement
delivered pursuant to this Section 12 may be relied upon by any prospective
purchaser or mortgagee of the Subleased Premises.

12. Sublease Subordinate. This Sublease Agreement shall be subject and
subordinate to any mortgage now or hereafter placed on the Main Premises, or any
portion or portions thereof or interest therein. This Section 13 shall be
self-operative and no further instrument of subordination shall be required.

13. Waiver of Subrogation. Any insurance carried by either party with respect to
the Main Premises (or any portion thereof and property therein or occurrence
thereon) shall, if it can be so written without additional premium, or with an
additional premium which the other party agrees to pay, include a clause or
endorsement denying to the insurer rights of subrogation against the other party
to the extent rights have been waived by the insured prior to occurrence of
injury or loss. Each party, notwithstanding any provisions of this Lease to the
contrary, hereby waives any rights or recovery against the other for injury or
loss due to hazards covered by insurance containing such clause or endorsement
to the extent of the indemnification received thereunder.

14. Access. Upon reasonable advance notice and at reasonable times, subject to
Sublessee's security practices and in such manner as to minimize interruption of
Sublessee's business operations, except in cases of emergency, in which case no
such notice shall be required, Sublessor or its representative may enter to view
the Subleased Premises to assure compliance with the terms of this Sublease
Agreement and for purposes of carrying out its maintenance and repair
obligations hereunder.

15. Assignment and Subletting. Sublessee shall not assign or Sublease the
Subleased Premises or any portion thereof, without the prior written consent of
Sublessor and Landlord, which consent may be withheld in the sole discretion of
either Sublessor or Landlord. Provided, however, that Sublessee may sublet a
portion of the Subleased Premises to Union Biometrica, Inc., upon prior written
notice to Sublessor and Landlord of such sublet no consent of either Landlord or
Sublessor shall be require, provided that such Sublease shall state that it
shall be wholly subject and subordinate to the terms of the Overlease and this
Sublease. Any such Sublease shall provide that it will terminate immediately
upon any default, beyond applicable cure periods, of Sublessee or Sublessor
under the Sublease and Lease, respectively.

16. Relationship between Sublessor and Sublessee. The parties hereto agree that
the relationship between Sublessor and Sublessee hereunder shall, except as
otherwise provided herein, be governed by the provisions of the Overlease as if
they were Landlord and Tenant under the Overlease.



17. Notices. All notices required or permitted hereunder shall be in writing and
shall be deemed duly served if and when delivered by hand, by overnight courier
(nationally recognized) or mailed by registered, or certified mail, postage
prepaid, return receipt requested, and addressed;

If to Sublessor:
T Cell Sciences, Inc.
115 Fourth Avenue
Dedham, MA 02192
Attention: General Counsel

with copies to:
Robert Orsi, Esq.
Tarlow, Breed, Hart, Murphy & Rodgers, P.C.
21 Custom House Street
Boston, MA 02110

If to Sublessee:
Repligen Corporation
117 Fourth Avenue
Needham, MA 02192
Attention: Walter Herlihy

with copies to:
Testa, Hurwitz & Thibeault, L.L.P.
125 High Street
Boston, MA 02110
ATTN: Real Estate Department

Any of the persons named in this Section 17 may change the address for notices
by written notice sent to each of the other persons at the addresses as set
forth herein.

18.Severability. If any provision of this Sublease Agreement shall to any extent
be determined by any court of competent jurisdiction to be invalid or
unenforceable for any reason. the parties agree to amend this Sublease Agreement
so as to effectuate the original intent of Sublessor and Sublessee.

19.Entire Agreement. Except for the services agreement to be executed between
the parties, this Sublease Agreement is the entire agreement between the parties
relating to the sublease or the Subleased Premises. This Sublease Agreement may
not be amended, altered or modified except by instrument in writing and executed
by Sublessor and Sublessee.

21.Governing Law. This Sublease Agreement shall be governed by and construed in
accordance with the laws of the Commonwealth of Massachusetts.

22.Bind and Inure. This Sublease Agreement shall be binding upon and inure to
the benefit of the parties hereto and their respective successors and assigns.



23.Defined Terms. All capitalized terms not otherwise defined herein shall have
the meaning scribed thereto in the Overlease.

24.Brokerage. Each of Sublessor and Sublessee represents and warrants that it
has not had any dealings with any realtors, brokers or agents in connection with
the negotiation of this Sublease. Each party shall defend, hold harmless and
indemnify the other and Landlord from and against any claims, liabilities,
losses or expenses, including reasonable attorneys fees, incurred by the other
or Landlord in connection with any breach of the foregoing representation and
warranty by such party.

25.Notwithstanding any contrary provision of this Sublease, in any instance
where a specific cure period is granted to Sublessor, as Tenant under the Lease,
before Sublessor is considered to be in default under the Lease, the Sublessee,
as tenant under this Sublease, shall be deemed to have the same cure period,
less three days, before Sublessee is considered in default beyond applicable
cure periods under this Sublease, except in the event of a failure in the
performance of any of Sublessee's obligations to pay any sum payable under this
Sublease, as to which the cure period shall be three days following receipt of
written notice of such default from Sublessor..

26.Sublessor's Representations and Warranties. Sublessor warrants and represents
that:

(i) The copy of the Overlease attached to this Sublease as Exhibit A is a
complete and accurate copy of the Overlease, which is in effect and has not been
amended;

(ii) To the best of Sublessor's knowledge, Landlord is not in default under the
Overlease, nor has any event occurred which, after any applicable notice and/or
the expiration of any grace period, shall constitute a default by Landlord under
the Overlease; (iii) To the best of Sublessor's knowledge, Sublessor is not in
default under the Overlease, nor has any event occurred which, after any
applicable notice and/or the expiration of any grace period, shall constitute a
default by Sublessor under the Overlease;

(iii) To the best of Sublessor's knowledge, Sublessor is not in default under
the Overlease, nor has any event occurred which, after any applicable notice
and/or the expiration of any grace period, shall constitute a default by
Sublessor under the Overlease;

(iv) All rent, additional rent and other charges due under the Overlease have
been paid as required through May 31, 1996;

(v) The Overlease is in full force and effect;

(vi) Landlord has represented to Sublessor that the Overlease has not been
encumbered by any prior transfer, assignment or encumbrance other than a
mortgage to Baybank, and to Metpath; and (vii) Sublessor has full and lawful
authority to enter into this Sublease, subject only to Landlord's consent.

c. Notwithstanding the foregoing, as between Sublessor and Sublessee, the terms
and conditions hereof shall be controlling whenever the terms and conditions of
the Overlease are contradictory to or inconsistent with the terms and conditions
hereof. Sublessor shall, upon written request by Sublessee, at Sublessee's cost
use diligent efforts to obtain from Landlord all permissions, consents,
approvals and documents needed by Sublessee to perform any act permitted by this
Sublease by reason of the incorporation by reference of the terms, covenants and
conditions of the Overlease, but Sublessee shall bear any monetary costs
associated with obtaining such consents.



EXECUTED under seal as of the date first hereinabove set forth.

Sublessor:

T CELL SCIENCES, INC.
/s/ Alan W. Tuck
By: Alan W. Tuck, President
Sublessee:

REPLIGEN CORPORATION
/s/ Avery W. Catlin
By: Avery W. Catlin,
Vice President, Finance