Document:

<PAGE>

                            FIRST AMENDMENT TO LEASE

         THIS FIRST AMENDMENT TO LEASE ("Amendment") is made as of this, 2nd day
of OCTOBER, 1997, by and between AQUILA BIOPHARMACEUTICALS, INC., a Delaware
corporation ("Landlord") and BIOMERIEUX VITEK, INC., a Missouri corporation
("Tenant").

                                   WITNESSETH:

         WHEREAS, Landlord and Tenant entered into a certain Lease ("Lease"),
dated October 22, 1996, pursuant to the terms of which Tenant is leasing from
Landlord certain space in an office building located at 1500 East Gude Drive and
in an office building located at 3-1/2 Taft Court, both of which buildings are
situated in Rockville, Montgomery County, Maryland;

         WHEREAS, Landlord and Tenant have agreed to modify certain of the
existing terms and conditions of the Lease, and to reconfirm certain other of
the existing terms and conditions of the Lease, all in the manner set forth
below.

         NOW, THEREFORE, in consideration of the covenants, conditions and
agreements contained herein, and of other good and valuable consideration, the
receipt of sufficiency of which are hereby acknowledged, Landlord and Tenant
agree as follows:

1.       DEFINITION OF TERMS. All capitalized terms not otherwise defined herein
shall have the meanings given them in the Lease.

2.       DESCRIPTION OF LEASED PREMISES.

         B.       GROSS LEASABLE AREA. Section 1(a)(2)of the Lease is hereby
amended and modified by deleting the Section in its entirety, and inserting in
lieu thereof the following:

         (2)      GROSS LEASEABLE AREA

         3,780 square feet in the office building located at 3-1/2 Taft Court;
         and 38,318 square feet in a portion of the office building located at
         1500 East Gude Drive.

3.       TENANT'S PROPORTIONATE SHARE OF CERTAIN COSTS AND EXPANSES.

         A.       TENANT'S PROPORTIONATE SHARE OF CERTAIN EXPANSES. Section 1(a)
(3) of the Lease is hereby amended and modified by deleting the section in its
entirety, and inserting in lieu thereof the following:
<TABLE>
<CAPTION>

         (3)                                              1500 E. Gude          3-1/2 Taft
                                                              Drive                Court
---------------------------------------------------------------------------------------------
        <S>                                                   <C>                  <C>
         A.       Proportionate Share -                        85%                  15%
         B.       R.E. Proportionate Share -                   85%                  34%
         C.       Insurance Proportionate Share -              85%                  34%

</TABLE>

<PAGE>

4.       MINIMUM ANNUAL RENT AND BASIC MONTHLY RENT.

         A.       MINIMUM ANNUAL RENT. Section 1(a)(6) of the Lease is hereby
amended and modified by deleting the section in its entirety, and inserting in
lieu thereof the following:

<TABLE>
<CAPTION>

         (6)                      1500 E. Gude           3-1/2 Taft                    Total for the
                                      Drive                 Court                     Leased Premises
--------------------------------------------------------------------------------------------------------
<S>                               <C>                   <C>                            <C>
Lease Year 1 -                     $440,657.00           $33,350.00                     $474,007.00
Lease Year 2 -                     $440,657.00           $33,350.00                     $474,007.00
Lease Year 3 -                     $440,657.00           $33,350.00                     $474,007.00
Lease Year 4 -                     $478,975.00           $36,250.00                     $515,225.00
Lease Year 5 -                     $478,975.00           $36,250.00                     $515,225.00
Lease Year 6 -                     $478,975.00           $36,250.00                     $515,225.00
Lease Year 7 -                     $478,975.00           $36,250.00                     $515,225.00
Lease Year 8 -                     $517,293.00           $39,150.00                     $556,443.00
Lease Year 9 -                     $517,293.00           $39,150.00                     $556,443.00
Lease Year 10 -                    $517,293.00           $39,150.00                     $556,443.00

</TABLE>

         B.       BASIC MONTHLY RENT. Section 1(a)(7) of the Lease is hereby
amended and modified by deleting the section in its entirety, and inserting in
lieu thereof the following:

<TABLE>
<CAPTION>

         (6)                      1500 E. Gude           3-1/2 Taft                    Total for the
                                      Drive                 Court                     Leased Premises
----------------------------------------------------------------------------------------------------------
<S>                               <C>                    <C>                           <C>
Lease Year 1 -                     $36,721.42             $2,779.16                     $39,500.58
Lease Year 2 -                     $36,721.42             $2,779.16                     $39,500.58
Lease Year 3 -                     $36,721.42             $2,779.16                     $39,500.58
Lease Year 4 -                     $39,914.58             $3,070.83                     $42,935.92
Lease Year 5 -                     $39,914.58             $3,070.83                     $42,935.92
Lease Year 6 -                     $39,914.58             $3,070.83                     $42,935.92
Lease Year 7 -                     $39,914.58             $3,070.83                     $42,935.92
Lease Year 8 -                     $43,107.75             $3,262.50                     $46,370.25
Lease Year 9 -                     $43,107.75             $3,262.50                     $46,370.25
Lease Year 10 -                    $43,107.75             $3,262.50                     $46,370.25

</TABLE>

         If and to the extent that any provision of this Lease reasonably
pertains to payment of Minimum Annual Rent and Basic Monthly Rent for one of
said portions of the Leased Premises (but not to both portions thereof), then
the amounts set forth in Sections 1(a)(6) and (7) of this Lease shall be
applied, as appropriate. Section 4(a) of the Lease shall be amended and modified
accordingly.

5.       DEMISING PLAN AND RELATED EXPENSES:

         Landlord and Tenant agree that Tenant shall have the right to
reconfigure the demising walls of the Gude Drive Premises on the first (1st)
floor of the office building in which the Gude Drive Premises is located, and
that the floor plan of said first (lst) floor of the Gude Drive

<PAGE>

Drive Premises is located, and that the floor plan of said first (1st) floor
of the Gude Drive Premises shall be as outlined and hatched on the attached
Exhibit B attached hereto and made a part hereof. All computations of Minimum
Annual Rent, Basic Monthly Rent, and the various proportionate share figures
set forth in Section 1 hereof have been made on the basis of said new
configuration of the first (lst) floor portion of the Gude Drive Premises.

6.       COUNTERPART COPIES. This Amendment may be executed in two or more
counterpart copies, all of which counterparts shall have the same course and
effect as if all parties hereto had executed a single copy thereof.

7.       CONTROLLING LANGUAGE. Insofar as the specific terms and provisions of
this Amendment purport to amended or modify or are in conflict with the specific
terms and provisions of the Lease, the terms and provisions of this Amendment
shall govern and control; in all other respects, the terms and provisions of the
Lease shall remain unmodified and in full force and effect.

8.       EFFECTIVE DATE. This Amendment shall be effective upon the execution of
this document by both parties hereto, as evidenced by each party's signature
below the signature line as set forth below.

         IN WITNESS WHEREOF, Landlord and Tenant have each caused this Amendment
to be duly executed, sealed and delivered, by its duly authorized attorney in
fact, as of the day and year first above written.

WITNESS/ATTEST:              Landlord: Aquila Biopharmaceuticals, Inc.

/s/ WITNESS                  By:        /s/   Stephen J. DiPalma
--------------------                    ----------------------------------------
                             Name:      Stephen J. DiPalma
                                        ----------------------------------------
                             Title:     Chief Financial Officer
                                        ----------------------------------------
                             Date:      October 2, 1997                   , 1997
                                        ----------------------------------

WITNESS/ATTEST:              Tenant: bioMerieux Vitek, Inc.

                             By:
--------------------                    ----------------------------------------
                             Name:
                                        ----------------------------------------
                             Title:
                                        ----------------------------------------
                             Date:                                        , 1997
                                        ----------------------------------

<PAGE>

6.       COUNTERPART COPIES. This Amendment may be executed in two or more
counterpart copies, all of which counterparts shall have the same course and
effect as if all parties hereto had executed a single copy thereof.

7.       CONTROLLING LANGUAGE. Insofar as the specific terms and provisions of
this Amendment purport to amended or modify or are in conflict with the specific
terms and provisions of the Lease, the terms and provisions of this Amendment
shall govern and control; in all other respects, the terms and provisions of the
Lease shall remain unmodified and in full force and effect.

8.       The provisions of paragraph 1 (Lease Modification) of the Closing
Agreement dated as of October 21, 1996 among Cambridge Biotech Corporation,
Aquila Biopharmaceuticals, Inc. and bioMeriux Vitek, Inc., regarding a
modification to the Lease, shall be deemed satisfied by the amendments set
forth herein, and of no further force and effect.

9.       EFFECTIVE DATE. This amendment shall be effective upon the execution of
this document by both parties hereto, as evidenced by each party's signature
below the signature line as set forth below.

         IN WITNESS WHEREOF, Landlord and Tenant have each caused this Amendment
to be duly executed, sealed and delivered, by its duly authorized attorney in
fact, as of the day and year first above written.

WITNESS/ATTEST:               Landlord: Aquila Biopharmaceuticals, Inc.

/s/ WITNESS                   By:        /s/   Stephen DiPalma
--------------------                     ---------------------------------------
                              Name:      Stephen DiPalma
                                         ---------------------------------------
                              Title:     CFO
                                         ---------------------------------------
                              Date:      October 29                       , 1997
                                         ---------------------------------

WITNESS/ATTEST:               Tenant: bioMerieux Vitek, Inc.

/s/ WITNESS                   By:        /s/  PJ Walsh
--------------------                     ---------------------------------------
                              Name:      Patrick J. Walsh
                                         ---------------------------------------
                              Title:     Vice President, Finance & CFO
                                         ---------------------------------------
                              Date:      October 29                       , 1997
                                         ---------------------------------

<PAGE>

                            Exhibit A, Demising Plan
                    (First Floor, for demising purposes only)

                                 [FLOOR PLAN]

<PAGE>

                  ASSIGNMENT OF LEASE AND ASSUMPTION AGREEMENT

         FOR VALUE RECEIVED, BIOMERIEUX VITEK, INC., a Missouri corporation
having an office at 595 Anglum Drive, Hazelwood, Missouri 63242-2395
("Assignor"), hereby assigns to CAMBRIDGE BIOTECH CORPORATION, a Delaware
corporation, having an office at 1500 East Gude Drive, Rockville, Maryland
("Assignee"), all of Assignor's right, title and interest as tenant under the
lease described in Exhibit A attached hereto and made a part hereof (the
"Lease"), to have and to hold the same unto Assignee, its successors and
assigns, from and after October 22, 1996 for the remainder of the term of the
lease (including any renewal, extension and other options therein contained),
subject to the rents, covenants, conditions and provisions therein contained.
Assignee hereby accepts the assignment of the Lease and assumes the performance
of all of the covenants and obligations of tenant under the Lease from and after
October 22, 1996 and hereby agrees to comply with all of the terms and
provisions thereof to be performed by tenant thereunder and all of the
obligations of Assignor thereunder from October 22, 1996. Assignor hereby
acknowledges pursuant to Section 14(a) of the Lease that Assignor shall remain
liable jointly and severally with Assignee for the due performance of all of the
terms, covenants, conditions and agreements contained in the Lease on Assignee's
part to be performed.

                    IN WITNESS WHEREOF, Assignor and Assignee have executed and
sealed this instrument as of October 22, 1996.

ATTEST:                                         BIOMERIEUX VITEK, INC.

/s/  Patrick J. Walsh                           By:   /s/  Philippe Archinard
-------------------------------------                 --------------------------
     Patrick J. Walsh                                 Philippe Archinard
     Vice-President and Treasurer                     President

ATTEST:                                         CAMBRIDGE BIOTECH CORPORATION

/s/  William J.T. Brown                         By:    /s/  Philippe Archinard
-------------------------------------                  -------------------------
     William J.T. Brown                                Philippe Archinard
     Counsel                                           President

Accepted and Agreed to
This 22nd day of October, 1996

AQUILA BIOPHARMACEUTICALS, INC.

By:  /s/ Aquila Biopharmaceuticals, Inc.               (Signed in counterpart)
   ----------------------------------

<PAGE>

STATE OF MISSOURI                       )
                                        ) ss.:
COUNTY OF ST. LOUIS                     )

         Before me, a Notary Public in and for said County and State, personally
appeared BIOMERIEUX VITEK, INC by PHILIPPE ARCHINARD, its PRESIDENT , who
acknowledged that [s]he did sign the foregoing instrument for and on behalf of
said corporation and by authority of its Board of Directors and that the same is
the free and corporate act of said corporation and the free act and deed of
him[her]self as such officer.

         IN TESTIMONY WHEREOF, I have hereunto set my hand and official seal
at___________, _____________ this 22 day of October, 1996.

                                                       /s/   NOTARY PUBLIC
                                                       -------------------------
                                                       Notary Public

                                                       [Image of Notary Seal]

STATE OF New York                       )
                                        ) ss.:
COUNTY OF New York                      )

         Before me, a Notary Public in and for said County and State, personally
appeared CAMBRIDGE BIOTECH CORPORATION by PHILIPPE ARCHINARD, its PRESIDENT,
who acknowledged that [s]he did sign the foregoing instrument for and on behalf
of said corporation and by authority of its Board of Directors and that the same
is the free and corporate act of said corporation and the free act and deed of
him[her]self as such officer.

         IN TESTIMONY WHEREOF, I have hereunto set my hand and official seal at
__________, ____________ this 25 day of October, 1996.

                                                       /s/   NOTARY PUBLIC
                                                       -------------------------
                                                       Notary Public

                                                       [Image of Notary Seal]

<PAGE>

                                    EXHIBIT A

Lease dated October 22, 1996 by and between AQUILA BIOPHARMACEUTICALS, INC. as
Landlord and BIOMERIEUX VITEK, INC., as Tenant of premises located at 3 1/2 Taft
Court and 1500 East Gude Drive, Rockville, Maryland<PAGE>

                           SUBLEASE ("SUBLEASE") dated as of December 17, 1998,
                  by and between CAMBRIDGE BIOTECH CORPORATION, the address of
                  which is c/o bioMerieux, Inc., 595 Anglum Drive, St. Louis,
                  Missouri 63042-2395 ("SUBLESSOR"), and CALYPTE BIOMEDICAL
                  CORPORATION, the address of which is 1440 Fourth Street,
                  Berkeley, California 94710 ("SUBLESSEE").

                                   WITNESSETH:

         WHEREAS, Sublessor is the assignee of bioMerieux, Inc. ("BMX"), lessee
under the Lease dated October 22, 1996, between ARE-1500 East Gude LLC, as
successor to Aquila Biopharmaceuticals, Inc., as Landlord, and BMX, as Tenant,
as amended through the date hereof (the "Master Lease"; capitalized terms used
and not otherwise defined herein have the respective meanings given them in the
Master Lease);

         WHEREAS, Sublessor has sold to Sublessee, pursuant to the Asset
Acquisition Agreement between the Sublessor and the Sublessee dated November 18,
1998 (the "Acquisition Agreement"), the Acquired Assets (as defined in the
Acquisition Agreement) located on the Leased Premises; and

         WHEREAS, Sublessor now wishes to sublease to Sublessee, and Sublessee
wishes to sublease from Sublessor, the Leased Premises on the following terms
and conditions;

         NOW, THEREFORE, Sublessor and Sublessee hereby agree as follows:

                                   I. ARTICLE

1.                 SUBLEASE. Upon and subject to the terms and conditions
hereinafter set forth, and subject to the terms and conditions of the Master
Lease, Sublessor subleases to Sublessee the Leased Premises.

1.                  TERM. The term of this Sublease shall commence on December
11, 1998 and end on December 10, 2000, or prior to such date upon one hundred
eighty (180) days prior written notice delivered by Sublessee. Sublessee may,
upon the expiration of the term of this Sublease on December 10, 2000 (but not
any earlier expiration), extend the original term of this Sublease for
successive periods of one year each by giving to Sublessor notice of each such
election at least one hundred eighty (180) days prior to the expiration of the
original term or then renewal term hereof, (but not to any date after the
Expiration Date); PROVIDED, HOWEVER, that Sublessee shall not have such right to
extend if it shall then be in default under the terms of the Sublease (at the
time of election) or if this Sublease has been earlier expired or terminated.

<PAGE>

3.                  RENT. The rent hereunder shall equal the Rent and the
Additional Rent.

                                   ARTICLE II

1.                  SUBORDINATION TO MASTER LEASE. This Sublease is and shall at
all times be subject and subordinate to the Master Lease, and notwithstanding
anything elsewhere herein to the contrary, upon the expiration or earlier
termination of the Master Lease this Sublease shall automatically and
simultaneously terminate.

2.                  INCORPORATION OF TERMS OF MASTER LEASE. In addition to the
terms and conditions set forth herein, and except as expressly modified herein,
the terms, conditions and respective obligations of Sublessor and Sublessee to
each other under this Sublease shall be the terms, conditions and respective
obligations of Lessor and Lessee to each other under the Master Lease, which
terms, conditions and obligations are hereby incorporated herein. Therefore, for
purposes of this Sublease, wherever in the Master Lease the word "Lessor" is
used, it shall be deemed to mean and refer to the Sublessor herein, wherever in
the Master Lease the word "Lessee" is used, it shall be deemed to mean and refer
to the Sublessee herein, and wherever in the Master Lease the words "Leased
Premises", are used, they shall be deemed to mean and refer to the subleased
property and the components thereof; PROVIDED, HOWEVER, that Sublessee shall
have no rights with respect to the option to purchase described in Section 38 of
the Master Lease.

3.                  ASSUMPTION BY SUBLESSEE. During the term of this Sublease,
and thereafter with respect to obligations which have arisen prior to the
termination or expiration of the term of this Sublease, Sublessee expressly
assumes and agrees to pay, perform and comply with for the benefit of Sublessor
and the Lessor under the Master Lease each and every payment and performance
obligation under the Master Lease with respect to Sublessee and the Subleased
Property.

4.                  ENVIRONMENTAL LIABILITIES.

a.                         Notwithstanding any other provision of this Sublease,
Sublessor shall indemnify and hold harmless the Sublessee and Sublessee's
officers, directors, employees, agents, affiliates, successors and assigns from
and against, any and all losses, liabilities, damages, costs and expenses
(including reasonable fees and disbursements of counsel and expenses of
investigation and defense), claims, or other obligations of any nature to the
extent attributable to (a) environmental conditions existing at the Leased
Premises prior to the execution of this Sublease which are attributable to the
actions of Sublessor while Sublessor occupied the Leased Premises (or are
actually known to the officers of Sublessor), (b) any violation of environmental
laws based on actions, failures to act, or conditions associated with the Leased
Premises during the time that Sublessor occupied the Leased Premises, or (c)
environmental conditions existing at the Leased Premises prior to the execution
of this Sublease which are not attributable to the actions of Sublessor while
Sublessor occupied the Leased Premises, to the extent that indemnification
therefor is paid to the Sublessor pursuant to existing agreements with

                                      -2-
<PAGE>

Aquila Biopharmaceutical Corporation (which indemnification shall be sought if
applicable in good faith by the Sublessor), or (d) any violation of
environmental laws based on actions, failures to act, or conditions not
associated with the Leased Premises during the time that Sublessor occupied the
Leased Premises, to the extent that indemnification therefor is paid to the
Sublessor pursuant to existing agreements with Aquila Biopharmaceutical
Corporation (which indemnification shall be so sought).

b.                         Notwithstanding any other provision of this Sublease,
Sublessee shall indemnify and hold harmless Sublessor and Sublessor's officers,
directors, employees, agents, affiliates, successors and assigns from and
against, any and all losses, liabilities, damages, costs and expenses (including
reasonable fees and disbursements of counsel and expenses of investigation and
defense), claims, or other obligations to the extent attributable to (a)
environmental conditions which are the result of actions of the Sublessee
subsequent to the execution of this Sublease, or (b) any violation of
environmental laws based on actions, failures to act, or conditions associated
with the Leased Premises during the Sublessee's occupation of the Leased
Premises.

5.                  EVENT OF DEFAULT. Any Event of Default under the Master
Lease shall constitute an Event of Default under this Sublease, and upon the
occurrence of any Event of Default under the Master Lease, Sublessor shall have
with respect to Sublessee and the Subleased Property all of the remedies
afforded the Lessor with respect to the Lessee and the Leased Property under the
Master Lease.

6.                  NOTICES. Except as required by law for the posting of
notices, all notices, requests, demands and other communications hereunder must
be in writing and shall be personally served or mailed (by registered or
certified mail, return receipt requested and postage prepaid), or delivered by a
national overnight delivery service such as Federal Express or D.H.L., or by
facsimile transmission addressed to the respective parties, as follows:

                  (a) if to Sublessor:

                  Cambridge Biotech Corporation
                  595 Anglum Drive
                  St. Louis, Missouri 63042-2395
                  Attention: President
                  Telecopier: (314) 731-7319

                  if to Sublessee:

                  Calypte Biomedical Corporation
                  1140 [sic] Fourth Street
                  Berkeley, California 94710
                  Attention: President
                  Telecopier: (510) 526-5381

                                      -3-
<PAGE>

         Any such mailing, delivery or other permitted service shall be deemed
to be complete on the day of the confirmed receipt or refusal thereof.

7.       MISCELLANEOUS.

a.                  SURVIVAL, CHOICE OF LAW. Anything contained in this Sublease
to the contrary notwithstanding, all claims against, and liabilities of,
Sublessee or Sublessor arising prior to any date of termination of this Sublease
shall survive such termination. If any late charges provided for in any
provision of this Sublease are based upon a rate in excess of the maximum rate
permitted by applicable law, the parties agree that such charges shall be fixed
at the maximum permissible rate. All the terms and provisions of this Sublease
shall be binding upon and inure to the benefit of the parties hereto and their
respective successors and assigns. The headings in this Sublease are for
convenience of reference only and shall not limit or otherwise affect the
meaning hereof. This Sublease shall be governed by and construed in accordance
with the laws of Maryland, except as to matters which under the laws of a State,
or under applicable procedural conflicts of laws rules, require the application
of laws of the State.

b.                  COUNTERPARTS. This Sublease may be executed in separate
counterparts, each of which shall be considered as original when each party has
executed and delivered to the other one or more copies of this Sublease.

c.                  ENTIRE AGREEMENT. There are no oral or written agreements or
representations between the parties hereto affecting this Sublease. This
Sublease supersedes and cancels any and all previous negotiations, arrangements,
representations, brochures, agreements and understandings, if any, between
Sublessor and Sublessee.

d.                  AMENDMENTS IN WRITING. Neither this Sublease nor any
provision hereof may be changed, waived, discharged or terminated except by an
instrument in writing signed by Sublessor and Sublessee

e.                  SEVERABILITY. If any provision of this Sublease or the
application of such provision to any person, entity or circumstance is found
invalid or unenforceable by a court of competent jurisdiction, such
determination shall not affect the other provisions of this Sublease and all
other provisions of this Sublease shall be deemed valid and enforceable.

f.                  SUCCESSORS. All rights and obligations of Sublessor and
Sublessee under this Sublease shall extend to and bind the respective heirs,
executors administrators and the permitted concessionaires, successors,
subtenants and assignees of the parties.

                                      -4-
<PAGE>

a.                  SUCCESSORS. All rights and obligations of Sublessor and
Sublessee under this Sublease shall extend to and bind the respective heirs,
executors administrators and the permitted concessionaires, successors,
subtenants and assignees of the parties.

         IN WITNESS WHEREOF, the parties have executed this Sublease by their
duly authorized signing officers.

                                        CAMBRIDGE BIOTECH CORPORATION

                                        By:  /s/ Philippe Sans
                                           -------------------------------------
                                                 Name: Philippe Sans
                                                 Title: President

                                        CALYPTE BIOMEDICAL CORPORATION

                                        By:       /s/  William A. Boeger III
                                           ----------------------------------
                                                 Name:
                                                 Title:

                                      -5-

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