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                                                                    EXHIBIT 10.9



                                                             CONTRACT NO. B-1303


                            VACA VALLEY BUSINESS PARK

                                  OFFICE LEASE



Landlord:

      Sepulveda Properties, Inc.

      a California corporation

Tenant:

      Biosource

      a California corporation

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                                      LEASE

      THIS LEASE, made as of the 15th day of October, 1987, by and between
Sepulveda Properties, Inc., a California corporation, ("Landlord"), and
Biosource, a California corporation ("Tenant"):

                                   WITNESSETH:

                                1. LEASE SUMMARY

      As used herein, the following terms shall have the meanings set forth
opposite them. Other terms may be defined in other parts of this Lease

      BUILDING: The building constructed by Landlord at 3333 Vaca Valley
      Parkway, Vacaville, California, including all land thereunder and all
      appurtenances thereto, such as entries, sidewalks, patios, curb areas,
      parking areas and landscaped areas.

      PREMISES: The interior space shown outlined in red on the floor plan of
      the Building attached hereto as Exhibit A, (containing approximately 9,304
      square feet); including the appurtenant right to use, in common with
      others, the lobby, entrances, sidewalks, curb areas, parking areas, and
      other public portions of the Building; but excluding all windows and the
      exterior surface of the outside walls of said space, all areas within said
      space, which are used in common by other tenants or for the benefit of
      other space within the Building and used for shafts, stacks, pipes,
      conduits, ducts, electric or other utilities, sinks and other Building
      facilities, and the use thereof and access thereto through said space for
      the purposes of maintenance, repairs and operation of the Building.

      VACA VALLEY BUSINESS PARK: The area developed or to be developed and shown
      outlined in red on Exhibit B hereto.

      TERM: 60 months.

      COMMENCEMENT DATE: December 15, 1987

      EXPIRATION DATE: December 14, 1992

      EXPENSE STOP: $930.40 per month

      BASE RENT: $7,992.14 per month (including Expense Stop) at the
      Commencement Date, to be adjusted as provided in paragraphs 3 and 4
      hereof.


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      TENANT'S SHARE: 17.9% of the Improved Area.

      PERMITTED USE(s): Office and laboratory use to the extent consistent with
      Exhibit D - Rules and Regulations, attached hereto.

      SECURITY DEPOSIT: $15,000 in accordance with paragraph 6, herein.

      GUARANTOR(s):

            Not applicable.

      LANDLORD'S ADDRESS:

            Sepulveda Properties, Inc.
            3333 Vaca Valley Parkway, Suite 100
            Vacaville, California 95688

      TENANT'S ADDRESS:

            Biosource
            562 Waddell Drive, Suite 5
            Sunnyvale, California 94089

      BROKER:  Lindquist Realty


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                           2. LEASE OF PREMISES; USES

      Landlord hereby leases the Premises to Tenant, and Tenant hereby leases
the Premises from Landlord, for the Term and subject to the provisions
hereinafter set forth. The Premises shall be used only for the Permitted Use(s).
Tenant shall in its use and enjoyment of the Premises observe and abide by, and
shall require each of its contractors, invitees and licensees to observe and
abide by, all laws, statutes, ordinances, rules and regulations, any certificate
of occupancy, and any recorded document affecting the Premises. Tenant shall not
do or permit to be done in or about the Premises anything which will in any way
increase the existing rate of or affect any fire or other insurance upon the
Building or any of its contents. Neither Tenant nor any of its employees,
invitees or licensees, shall do or permit anything to be done in or about the
Premises which will in any way obstruct or interfere with the rights of other
tenants of the Building, or injure or annoy them, nor shall Tenant or any of its
employees, invitees or, licensees, cause, maintain or permit any nuisance in, on
or about the Premises or commit or suffer to be committed any waste in, on or
about the Premises. Tenant shall not bring into the Building any furniture,
equipment, materials or other objects which overload the Building or any portion
thereof.

                       3. TERM; COMPLETION OF IMPROVEMENTS

      Landlord shall constructor install in the Premises, improvements in
accordance with the Work Agreement attached hereto as Exhibit C. It is
understood that Landlord shall be constructing the improvements in two phases -
the first phase is to include approximately 4,204 square feet of office space
and the second phase is to include approximately 5,100 square feet of research
and development space. The Term shall begin on the Commencement Date, and,
unless sooner terminated as hereinafter provided, shall end on the Expiration
Date. If Landlord, for any reason whatsoever, cannot deliver possession of the
Premises (with the first phase of the improvements covered by Exhibit C
substantially complete) to Tenant on the Commencement Date, this Lease shall not
be void or voidable, nor shall Landlord be liable to Tenant for any loss or
damage resulting therefrom (unless such improvements are not substantially
complete by the 60th day following Landlord's receipt of Tenant's written
approval of Tenant's space plan for the Premises), but in that event, subject to
any contrary provisions in Exhibit C attached hereto, all Rent including the
Expense Stop and Operating Expenses shall be waived for the period between the
Commencement Date and the first to occur of M the time when Landlord can deliver
possession (per Exhibit C); or (ii) the time when Tenant occupies the Premises.
This Lease shall neither be void nor voidable nor shall Landlord be liable to
Tenant for any loss or damage resulting from Landlord's not substantially
completing the improvements for the second phase, concurrently with the first
phase improvements. No delay in delivery of possession shall extend the Term or
the Expiration Date except at the option of Tenant, by written notice to
Landlord on or before the thirtieth day after the improvements are substantially
complete. The phrase "substantially complete" shall mean that the improvements
are functionally usable for the permitted use described herein and either (a)
Landlord gives Tenant written notice thereof; or (b) a certificate of occupancy
for the Premises has been issued by the appropriate government office.


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                                  4. BASE RENT

      Tenant shall pay Expense Stop and Base Rent, adjusted from time to time as
described below, to the Landlord on the first day of each calendar month or part
thereof during the Term. If either the Term or rent commences on a day other
than the first day of a calendar month or ends on a day other than the last day
of a calendar month, then the rent for the fractional month shall be
appropriately prorated. Expense Stop and Base Rent shall be paid to Landlord,
without deduction or offset, at Landlord's Address, or to such other person or
at such other place as Landlord may from time to time designate in writing.
Effective as of the third (3rd) anniversary of the Commencement Date, the Base
Rent shall be adjusted according to the proportion of (1) the Consumer Price
Index for All Urban Consumers for San Francisco-Oakland, California, as
published by the Bureau of Labor Statistics of the United States Department of
Labor ("CPI-U") published most recently prior to such third anniversary date, to
(1i) CPI-U published most recently prior to the Commencement Date; provided,
however, that as a result of such adjustment the Base Rent shall not be
increased by more than twelve percent (12%), nor reduced. On the fourth (4th)
anniversary of the Commencement Date, the Base Rent shall again be adjusted
according to the proportion of (A) CPI-U published most recently prior to such
fourth (4th) anniversary date, to (B) CPI-U published most recently prior to the
date of the last adjustment of Base Rent; provided, however, that as a result of
such adjustment the Base Rent as previously adjusted shall not be increased by
more than four percent (4%), nor reduced. In the event of a change in the manner
or method of computing CPI-U, Landlord and Tenant shall endeavor to agree on a
mutually acceptable substitute price index and a date as of which such
substitute price index shall take effect. Any and all additional amounts payable
by Tenant to Landlord hereunder whether or not identified as additional rent and
any amount payable by Tenant to Landlord under the Work Agreement attached
hereto as Exhibit C, shall be considered additional rent. Base Rent shall be
earned in proportion to the amount of square footage of the Premises which is
substantially complete (in accordance with Exhibit C) or when occupied,
whichever is first in time.

                        5. OPERATING COSTS ADJUSTMENTS

      (a) During the Term hereof, Tenant shall pay to Landlord as additional
rent the Tenant's Share of the total dollar increase, if any, in "Operating
Expenses" as hereinafter defined, paid or incurred by Landlord, over the Expense
Stop. As used in this Lease, "Operating Expenses" means all costs of labor
including independent contractors, together with wages, salaries, payroll
burden, taxes and applicable benefits, materials, supplies and services for the
management, operation maintenance, protection and repair of the Building,
including, without limitation, water and sewer charges; garbage and waste
disposal; license, permit and inspection fees; air conditioning, ventilation,
janitorial and cleaning service in the Building; plumbing service; maintenance,
repair and service contracts; insurance including Landlord's internal allocated
cost should Landlord elect to self-insure; supplies, materials, tools and
equipment; all costs and expenses of contesting by appropriate legal proceedings
any matter concerning operating or managing the Building or the amount or
validity of any property taxes levied against the Building; depreciation on all
personal property, fixtures and equipment (including window washing machinery)
used in the management, operation, maintenance and repair of the Building;
depreciation on exterior window coverings provided by Landlord and carpeting in
public corridors and common areas; expenses of operating and maintaining areas
within the Vaca


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Valley Business Park which are not rentable or appurtenant to rentable
improvements and which are properly allocable to the Building; real and personal
property taxes (and any tax levied wholly or partly in lieu thereof),
assessments, sewer rents, or utility rates or charges levied by any governmental
authority (local, state or Federal) against the Building and any personal
property used in its operation; provided, however, that Operating Expenses shall
not include taxes covered under paragraph 26, hereof, gas, electricity, and
water service if metered separately to the Premises, janitorial service for the
Premises (unless paid for by Landlord), depreciation on the Building (except as
specified above), costs of tenants' improvements, real estate brokers'
commissions, interest and capital items (except the cost of capital improvements
made after the Commencement Date which reduce any item of Operating Expenses,
amortized over a reasonable period, together with interest at the rate of ten
per cent (10%) per annum on the unamortized portion of such cost). The
determination of Operating Expenses and their allocation shall be in accordance
with generally accepted accounting principles applied on a consistent basis. If
the Building is not fully rented during all or a portion of any calendar year,
Landlord may make an appropriate adjustment of the Operating Expenses for such
year, employing sound accounting and management principles to determine the
amount of Operating Expenses that would have been paid or incurred by Landlord
had the Building been fully rented, and the amount so determined shall be deemed
to have been the amount of Operating Expenses for such year.

      (b) During December of each calendar year or as soon thereafter as
practicable, Landlord shall give Tenant written notice of Landlord's estimate of
the additional rent payable under subparagraph 5(a) hereof for the ensuing
calendar year. On or before the first day of each month during the ensuing
calendar year, Tenant shall pay to Landlord one twelfth (1/12) of such estimated
additional rent, provided that if such notice is not given in December, Tenant
shall continue to pay on the basis of the prior year's estimate until the month
after such notice is given. If at any time it appears to Landlord that the
additional rent payable under subparagraph 5(a)hereof for the current calendar
year will vary from its estimate by more than twenty percent (20 %), Landlord
shall, by written notice to Tenant, revise its estimate for such year, and
subsequent payments by Tenant for such year shall be based upon such revised
estimate.

      (c) Within ninety (90) days after the end of each calendar year or as soon
after such ninety (90) day period as practicable, Landlord shall deliver to
Tenant a statement of additional rent payable under subparagraph 5(a) hereof for
such calendar year. Such statement shall be final and binding upon Landlord and
Tenant unless Tenant makes a written request of Landlord within thirty (30) days
of receipt of such statement, to review Landlord's computation of the Operating
Expenses. If such statement shows an amount owing by Tenant that is less than
the estimated payments for such calendar year previously made by Tenant,
Landlord shall credit the excess to the next succeeding monthly installment of
rent. If such statement shows an amount owing by Tenant that is more than the
estimated payments for such calendar year previously made by Tenant, Tenant
shall pay the deficiency to Landlord within thirty (30) days after delivery of
such statement.

      (d) If, for any reason other than the default of Tenant, this Lease shall
terminate on a day other than the last day of a calendar year, the additional
rent payable by Tenant applicable to the calendar year in which such termination
shall occur shall be prorated on the basis which the number of days from the
commencement of such calendar year to and including such date of


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termination bears to three hundred sixty-five (365).

                             6. SECURITY DEPOSIT

      Tenant has deposited the Security Deposit with Landlord. The Security
Deposit shall be held by Landlord as security for the faithful performance and
observance by Tenant of all of its obligations under this Lease. In the event
Tenant fails to perform or observe any of its obligations under this Lease then
Landlord shall apply the Security Deposit, or so much thereof as may be
necessary, to compensate for any such failure by Tenant. Such application by
Landlord shall not constitute a waiver of any default by Tenant or failure to
perform an obligation under this Lease. If Landlord applies the Security Deposit
or any part thereof to remedy any such failure by Tenant, then Tenant shall
immediately pay to Landlord the sum necessary to restore the Security Deposit to
the full amount specified in paragraph 1 hereof. Any remaining portion of the
Security Deposit shall be returned to Tenant, without interest, upon termination
of this Lease. In addition thereto, Landlord shall pay to Tenant interest on the
Security Deposit which interest shall be computed by multiplying the lowest
balance of the Security Deposit during any lease year by five percent (5%) per
annum. The interest shall not be compounded and shall be deemed to be earned and
payable on the Expiration Date of the Lease. Upon termination of the original
Landlord's or any successor landlord's interest in the Premises or the Building,
the original Landlord or such successor landlord shall be relieved of further
liability with respect to the Security Deposit upon the original Landlord's or
such successor landlord's complying with California Civil Code section 1950.7.

                                   7. SERVICES

      (a) Landlord shall maintain the public and common areas of the Building,
such as lobbies, stairs, corridors, restrooms, landscaping and parking lot in
reasonably good order and condition (except for damage occasioned by the act of
Tenant or employees, invitees or licensees of Tenant, which damage shall be
repaired by Landlord at Tenant's expense).

                            8. RULES AND REGULATIONS

      Tenant shall faithfully observe and comply with the Rules and Regulations
attached as Exhibit D to this Lease and with all revisions thereof and additions
thereto issued in writing by Landlord. Landlord shall use its best efforts to
seek the compliance of other tenants of the Building, with such rules and
regulations.

                                   9. REPAIRS

      By entry hereunder Tenant acknowledges the Premises as being in the
condition in which Landlord is obligated to deliver the Premises, subject to the
completion or correction of minor items which do not materially impair the
usability of the Premises by Tenant and subject to latent defects in the
materials and construction of the improvements completed by Landlord for which
Landlord receives written notice from Tenant within one (1) year of substantial
completion of the improvements. Tenant shall, at all times during the term
hereof and at Tenant's sole cost and expense, keep the Premises and every part
thereof in good condition and repair, ordinary wear and tear and damage thereto
by fire, earthquake, act of God or the elements excepted, unless such a damage
is caused by Tenant or any of its contractors, invitees or licensees. Tenant
hereby


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waives all rights under California Civil Code section 1941 and all rights to
make repairs at the expense of Landlord or in lieu thereof to vacate the
Premises as provided by California Civil Code section 1942 or any other law,
statute or ordinance now or hereafter in effect. Tenant shall at the end of the
term hereof surrender to Landlord the Premises and all alterations, additions
and improvements thereto in the same condition as when received, ordinary wear
and tear and damage by fire, earthquake, act of God or the elements excepted,
unless such damage is caused by Tenant or any of its contractors, invitees or
licensees. Landlord has no obligation and has made no promise to alter, improve,
repair, decorate or paint the Premises, except as specified in Exhibit C. No
express representation respecting the condition of the Premises or the Building
has been made to Tenant by Landlord, except as specifically herein set forth.

                                 10. ALTERATIONS

      Tenant shall not alter or improve the Premises, or attach any fixtures or
equipment thereto without Landlord's prior written consent which shall not be
unreasonably delayed or withheld. Any alterations or improvements (except the
initial improvements covered by Exhibit C) to the Premises consented to by
Landlord shall be made by Tenant at Tenant's sole cost and expense. The
contractor or person selected by Tenant to make such alterations or improvements
must be approved in writing by Landlord prior to commencement of any work.
Landlord shall have the right to require that any such contractor hired by
Tenant shall, prior to commencing work in the Premises, provide Landlord with a
performance bond and a labor and materials payment bond in the amount of the
contract price for the work naming Landlord and Tenant (and any other person
designated by Landlord) as co-obligees. All alterations, additions, fixtures and
improvements, including improvements made pursuant to Exhibit C, made in or upon
the Premises either by Tenant or Landlord shall immediately become Landlord's
property and, at the end of the Term hereof (subject to the limitations
pertaining to trade fixtures as indicated on Exhibit G attached hereto), shall,
at Landlord's option, either remain on the Premises without compensation to
Tenant or be removed by Landlord for Tenant's account. Tenant shall reimburse
Landlord for the cost of removal (including the cost of repairing any damage to
the Premises or the Building caused by removal of such improvements and a
reasonable charge for Landlord's overhead) within ten (10) days after receipt of
a statement therefor.

                          11. DESTRUCTION OR DAMAGE

      If the Premises or the Building is damaged by fire or other casualty,
Landlord shall forthwith repair the same, subject to the provisions of this
paragraph 11 hereinafter set forth, provided such repairs can, in Landlord's
opinion, be made within sixty (60) days following such damage, and this Lease
shall remain in full force and effect. If such repairs cannot, in Landlord's
opinion, be made within such sixty (60) day period, Landlord at its option shall
by written notice to Tenant given within sixty (60) days after the date of such
fire or other casualty either (1) elect to repair or restore such damage, this
Lease continuing in full force and effect, or (2) terminate this Lease as of a
date specified in such notice, which date shall not be less than thirty (30)
days after the date such notice is given. If such fire or other casualty shall
have damaged the Premises or common areas necessary to Tenant's occupancy, and
if such damage is not the result of the negligence or willful misconduct of
Tenant or Tenant's employees, contractors, invitees or licensees, then during
the period the Premises are rendered unusable by such damage for the Permitted
Uses stated herein, Tenant shall be entitled to a reduction in rent in the
proportion that


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the area of the Premises rendered unusable by such damage bears to the total
area of the Premises. Landlord shall not be required to repair any injury or
damage or to make any repairs or replacements of any improvements installed in
the Premises by or for Tenant, other than Landlord's work under Exhibit C, and
Tenant shall, at Tenant's sole cost and expense, repair and restore its portion
of such improvements. If more than fifty percent (50%) of the Premises is
rendered unusable by fire or other casualty and Landlord advises Tenant that in
Landlord's opinion such damage cannot be repaired within sixty (60) days, Tenant
may, upon notice to Landlord, terminate this Lease. A total destruction of the
Building shall automatically terminate this Lease. Tenant hereby waives
California Civil Code sections 1932(2) and 1933(4) providing for termination of
hiring upon destruction of the thing hired.

                 12. INDEMNIFICATION AND INSURANCE; SUBROGATION

      (a) Tenant hereby waives all claims against Landlord for damage to any
property or injury, illness or death of any person in, upon or about the
Premises and/or the Building arising at any time and from any cause whatsoever
other than by reason of the active negligence or willful act of Landlord, its
employees or contractors. Tenant shall hold Landlord harmless and defend
Landlord against any and all claims or liability for any damage to any property
or injury, illness or death of any person: (1) occurring in or about the
Premises or any part thereof arising at any time and from any cause whatsoever
other than by reason of the active negligence or willful act of Landlord, its
employees or contractors, and (2) occurring in or about any part of the Building
other than the Premises when such damage, injury, illness or death shall have
been caused in whole or in part by the act, neglect, omission or fault of
Tenant, its agents, servants, employees, contractors, invitees or licensees. The
provisions of this paragraph 12 shall survive the termination of this Lease.

      (b) Tenant shall, at its sole cost and expense, obtain and keep in force
during the Term fire and extended coverage insurance on Tenant's improvements,
fixtures, furnishings and equipment in and upon the Premises in an amount not
less than one hundred per cent (100%) of the full replacement cost (without
deduction for depreciation) thereof. All amounts received from the insurance
specified in this subparagraph 12(b) shall first be applied to the payment of
the cost of repair or replacement of any of Tenant's improvements, fixtures,
furnishings and equipment that were damaged or destroyed, or, if this Lease
terminates prior to such repair or replacement being made, paid over to Landlord
to the extent that the improvements or fixtures damaged or destroyed would have
become Landlord's property pursuant to paragraph 10 hereof.

      (c) Tenant shall, at its sole cost and expense, obtain and keep in force
during the Term hereof M Comprehensive or Commercial General Liability Insurance
(Bodily Injury and Property Damage), including Contractual Liability to cover
liability assumed under this Lease, with a limit of liability of not less than
one million dollars ($1,000,000) per occurrence for injury to, illness of, or
death of persons or for damage to property occurring in, upon or about the
Premises or the Building. All such insurance shall insure the performance by
Tenant of the indemnity agreement set forth in subparagraph 12(a) hereof.

      (d) All insurance required under this paragraph 12 and all renewals
thereof shall be issued by such responsible companies qualified to do and doing
business in the State of California as may be approved by Landlord. Each policy
shall expressly provide that the policy


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shall not be cancelled or altered without thirty (30) days' prior written notice
to Landlord. All insurance under this paragraph 12 shall name Landlord as an
additional insured, shall be primary and noncontributing with any insurance
which may be carried by Landlord, and shall expressly provide that Landlord,
although named as an insured, shall nevertheless be entitled to recover under
the policy for any loss, injury or damage to or suffered by Landlord, its
employees and contractors. Upon the issuance thereof, each such policy or a
duplicate or certificate thereof shall be delivered to Landlord for its
retention. In the event that Tenant shall fall to insure or shall fail to
furnish to Landlord upon notice to do so, any such policy, duplicate policy or
certificate as herein required, Landlord may from time to time effect such
insurance for the benefit of Tenant or Landlord or both of them for a period not
exceeding one year, and any premium paid by Landlord shall be recoverable from
Tenant as additional rent on demand.

      (e) Tenant waives on behalf of its insurers under all policies of fire,
theft, public liability, workers' compensation and other insurance now or
hereafter existing during the Term hereof and purchased by it insuring or
covering the Premises, or any portion or any contents thereof, or any operations
therein, all rights of subrogation which any insurer might otherwise have to any
claims of Tenant against Landlord. Landlord waives on behalf of its insurers
under all policies of fire, theft, public liability, workers' compensation and
other insurance now or hereafter existing during the term hereof and purchased
by it insuring or covering the Building or any portion or the contents thereof,
or any operations therein, all rights of subrogation which any insurer might
otherwise have to any claims of Landlord against Tenant in excess of the limits
of any insurance Tenant is required to carry pursuant to this paragraph.
Landlord and Tenant shall each, prior to or immediately after the execution of
this Lease, procure from each of the insurers under all policies of fire, theft,
public liability, workers' compensation and other insurance now or hereafter
existing during the term hereof and purchased by either of them insuring or
covering the Building or the Premises, or any portion or any contents thereof,
or any operations therein, a waiver of all rights of subrogation which the
insurer might otherwise have as against the other, to the extent required by
this subparagraph 12(e).This subparagraph shall not be construed to require of
Landlord or Tenant any insurance coverage not otherwise required by this Lease
nor to waive any rights of recovery that either Landlord or Tenant may have
directly against the other to the extent that any loss or damage giving rise to
any such right of recovery is not actually covered by insurance.

                          13. ASSIGNMENT AND SUBLETTING

      Tenant shall not, without the prior written consent of Landlord, which
consent shall not be unreasonably withheld, assign or hypothecate this Lease or
any Interest herein or sublet the Premises or any part thereof, or permit the
use or occupancy of the Premises by any person other than Tenant, its wholly
owned subsidiaries, employees, invitees or licensees. A consent by Landlord to
any one assignment or subletting shall not constitute a consent to any other or
subsequent assignment or subletting. Tenant shall provide Landlord with a copy
of any assignment of this Lease or any sublease of the Premises and a copy of
any document pursuant to which any such assignment or sublease may be made.
Tenant shall pay or deliver over to Landlord, as additional rent hereunder, any
and all consideration received by Tenant in respect of any such assignment and
any and all amounts received under any such sublease in excess of the rent due
under this Lease attributable to such portion of the Premises as is covered by
such sublease and any and all other consideration received by Tenant in respect
of any such sublease.


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                          14. LANDLORD'S RIGHT OF ENTRY

      Landlord may enter the Premises at any reasonable time and upon notice to
Tenant except in the event of an emergency, to (i) inspect the Premises, (ii)
exhibit the Premises to prospective purchasers, lenders or tenants, (Iii)
determine whether Tenant is complying with all its obligations hereunder, (iv)
supply janitorial services, (v) post notices, (vi) make repairs, (vii) decorate,
remodel, repair, alter or otherwise prepare the Premises for reoccupancy if
Tenant vacates the Premises prior to the expiration of the Term, and (viii) take
any other measures, including inspections, repairs, alterations, additions and
improvements to the Premises or to the Building as may be necessary or desirable
for the safety, protection or preservation of the Premises, the Building or the
Vaca Valley Business Park. Any such entry shall be for a reasonable period only
and, if Tenant has not vacated the Premises, cause as little interference to
Tenant as reasonably possible. Tenant hereby waives any claim for damages for
any injury or inconvenience to or interference with Tenant's business, any loss
of occupancy or quiet enjoyment of the Premises or any other loss occasioned by
such entry. Landlord shall at all times have and retain a key with which to
unlock the Premises, and Landlord shall have the right to use any and all means
which Landlord may deem proper in an emergency in order to obtain entry to the
Premises. Any entry to the Premises obtained by Landlord by any of said means,
or otherwise, shall not under any circumstances be construed or deemed to be a
forcible or unlawful entry into or a detainer of the Premises or an eviction,
actual or constructive, of Tenant from the Premises, or any portion thereof.

                                    15. LIENS

      Tenant shall keep the Premises and the Building free from any liens
arising out of any work performed, materials furnished or obligations incurred
by Tenant. Landlord shall have the right to post and keep posted on the Premises
any notices that may be provided by law or which Landlord may deem to be proper
for protection from such liens, or to take any other action necessary to remove
or discharge liens or encumbrances at the sole expense of Tenant.

                               16. EMINENT DOMAIN

      If all or any part of the Premises shall be taken as a result of the
exercise of the power of eminent domain or agreement in lieu thereof, this Lease
shall terminate as to the part so taken as of the date of taking, and, in the
case of a partial taking, either Landlord or Tenant shall have the right to
terminate this Lease as to the balance of the Premises by giving written notice
to the other within thirty (30) days after such date. In the event of any
taking, Landlord shall be entitled to any and all compensation, damages, income,
rent, awards or interest therein whatsoever which may be paid or made in
connection therewith, and Tenant shall have no claim against Landlord for the
value of any unexpired term of this Lease or otherwise. In the event of a
partial taking of the Premises which does not result in a termination of this
Lease, the monthly rent thereafter to be paid shall be equitably reduced.

                              17. EVENTS OF DEFAULT

      The occurrence of any one or more of the following events ("Event of
Default") shall constitute a breach of this Lease by Tenant:


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      (a) If Tenant shall fail to pay any rent, or additional rent, or any other
sum or charge payable by Tenant hereunder, when and as the same becomes due and
payable; or

      (b) If Tenant shall fail to perform or observe any other term or provision
hereof or of the rules and regulations specified in paragraph 8 hereof, and such
failure shall continue for more than ten (10) days; or

      (c) If Tenant shall make a general assignment for the benefit of
creditors, or shall admit in writing its inability to pay its debts as they
become due, or shall file a petition in bankruptcy or shall be adjudicated a
bankrupt or insolvent; or

      (d) If this Lease or any estate of Tenant hereunder shall be levied upon
under any attachment or execution and such attachment or execution is not
vacated within ten (10) days; or

      (e) If Tenant shall abandon the Premises.

                                  18. REMEDIES

      (a) If an Event of Default shall occur, Landlord at any time thereafter
may give a written termination notice to Tenant and on the date specified in
such notice, Tenant's right to possession shall terminate and this Lease shall
terminate. Upon such termination Landlord may recover from Tenant:

            (i)   The worth at the time of award of the unpaid rent which had
                  been earned at the time of termination;

            (ii)  The worth at the time of award of the amount by which the
                  unpaid rent which would have been earned after termination
                  until the time of award exceeds the amount of such rental loss
                  that Tenant proves could be reasonably avoided;

            (iii) The worth at the time of award of the amount by which the
                  unpaid rent for the balance of the Term after the time of
                  award exceeds the amount of such rental loss that Tenant
                  proves could be reasonably avoided; and

            (iv)  Any other amount necessary to compensate Landlord for all the
                  detriment proximately caused by Tenant's failure to perform
                  its obligations under this Lease.

For the purpose of determining unpaid rent under clause (i), (ii) and (iii)
above, the monthly rent reserved in this Lease shall be deemed to be the sum of
the Base Rent due under paragraph 4 hereof and the additional rent last payable
by Tenant under paragraph 5 hereof.

      (b) Even though Tenant has breached this Lease, this Lease shall continue
in effect for so long as Landlord does not terminate Tenant's right to
possession pursuant to subparagraph (a) above, and Landlord may enforce all its
rights and remedies under this Lease, including the right to recover the rent as
it becomes due. Acts of maintenance or preservation or efforts to relet the
Premises or the appointment of a receiver upon initiative of Landlord to protect
Landlord's


                                       11
<PAGE>   13
interest under this Lease shall not constitute a termination of Tenant's right
to possession unless written notice of termination is given by Landlord to
Tenant.

      (c) If Tenant shall fail to pay any sum of money, other than rent,
required to be paid by it hereunder or shall fail to perform any other act on
its part to be performed hereunder, Landlord may, but shall not be obligated so
to do, and without waiving or releasing Tenant from any obligations of Tenant,
make any such payment or perform any such other act on Tenant's part to be made
or performed as in this Lease provided. All sums so paid by Landlord and all
necessary Incidental costs shall be deemed additional rent hereunder and shall
be payable to Landlord on demand. Landlord shall have (in addition to any other
right or remedy of Landlord) the same rights and remedies in the event of the
nonpayment thereof by Tenant as in the case of default by Tenant in the payment
of rent.

                        19. SUBORDINATION TO MORTGAGES

      At the option of Landlord, this Lease shall be subject and subordinate at
all times to the lien of all mortgages and deeds of trust in any amount or
amounts whatsoever which may now exist or hereafter be placed on or against the
Building or on or against Landlord's interest or estate therein and to any
ground or other underlying lease, all without the necessity of having further
instruments executed by Tenant to effectuate such subordination. Notwithstanding
the foregoing, in the event of a foreclosure of any such mortgage or deed of
trust or the termination of any such lease, this Lease shall not be terminated,
nor will the possession of Tenant hereunder be disturbed, if Tenant shall not
then be in default hereunder, and Tenant shall attorn to the purchaser at such
foreclosure or the landlord under any such lease. Tenant agrees to execute,
acknowledge and deliver upon demand such further instruments evidencing such
subordination of this Lease to the lien of any such mortgages or deeds of trust
or to any such lease as may reasonably be required by Landlord, its lender, its
landlord, or the title insurer of its lender or landlord; provided, however,
that Tenant's covenant to subordinate this Lease to mortgages or deeds of trust
or ground or other underlying leases hereafter executed is conditioned upon each
such senior instrument containing the commitment specified in the preceding
sentence.

                               20. ATTORNEYS' FEES

      In the event of any legal or equitable action between Landlord and Tenant
to enforce any provision of this Lease or to protect or establish any right or
remedy of either Landlord or Tenant hereunder, the unsuccessful party shall pay
to the prevailing party all costs and expenses, including reasonable attorneys'
fees, incurred in such action or proceeding and in any appeal in connection
therewith by such prevailing party, and any such costs, expenses and attorneys'
fees shall be included in and as a part of any such judgment entered in favor of
such prevailing party.

                                    21. SALE

      In the event the original Landlord hereunder, or any successor owner of
the Building, shall sell or convey the Building, all liabilities and obligations
on the part of the original Landlord accruing thereafter shall terminate, and
thereupon all such liabilities and obligations shall be binding upon the new
owner. Tenant agrees to attorn to such new owner.


                                       12
<PAGE>   14
                            22. ESTOPPEL CERTIFICATE

      At any time and from time to time but on not less than ten (10) days'
prior written request by Landlord, Tenant shall execute, acknowledge and deliver
to Landlord, promptly upon request, a certificate certifying:

      (a) that this Lease is unmodified and in full force and effect (or, if
there have been modifications, that this Lease is in full force and effect as
modified, and stating the date and nature of each modification);

      (b) the date, if any, to which rent and other sums payable hereunder
have been paid;

      (c) that, except as stated, no notice has been received by Tenant of any
default which has not been cured, except as to defaults specified in such
certificate;

      (d) that, except as stated, Landlord is not in default hereunder; and

      (e) such other matters as may be reasonably requested by Landlord, any
actual or prospective purchaser or mortgage lender.

Any such certificate may be relied upon by any actual or prospective purchaser,
mortgagee or beneficiary under any deed of trust encumbering, or landlord under
any ground or other underlying lease covering the Building or any part thereof.

                       23. NO LIGHT, AIR OR VIEW EASEMENT

      Any diminution or shutting off of light, air or view by any structure
which may be erected on lands adjacent to the Building shall in no way affect
this Lease or impose any liability on Landlord.

                                24. HOLDING OVER

      If, without objection by Landlord, Tenant holds possession of the Premises
after expiration of the Term, Tenant shall become a tenant from month to month
upon the terms herein specified at two hundred percent (200%) of the then
prevailing monthly rent paid by Tenant at the expiration of the Term pursuant to
the provisions of paragraphs 4 and 5 hereof, payable in advance on or before the
first day of each month. Each party shall give the other written notice at least
thirty (30) days prior to the date of termination of such monthly tenancy of its
Intention to terminate such tenancy.

                                 25. ABANDONMENT

      if Tenant shall abandon or surrender the Premises, or be dispossessed by
process of law or otherwise, any personal property belonging to Tenant and left
on the Premises shall be deemed to be abandoned, and, at the option of Landlord,
Landlord may sell or otherwise dispose of such personal property in any
commercially reasonable manner.


                                       13
<PAGE>   15
                           26. TAXES PAYABLE BY TENANT

      In addition to the Base Rent and additional rent and other charges to be
paid by Tenant hereunder, Tenant shall reimburse Landlord upon demand for any
and all of the following, if payable by Landlord, whether or not now customary
or within the contemplation of the parties hereto:

      (a) any and all taxes imposed upon, measured by or reasonably attributable
to the cost or value of Tenant's equipment, furniture, fixtures and other
personal property located in the Premises or by the cost or value of any
leasehold improvements made in or to the Premises by or for Tenant, other than
Building Standard Work under Exhibit C, regardless of whether title to such
improvements shall be in Tenant or Landlord;

      (b) any and all taxes imposed upon, measured by or reasonably attributable
to the Base Rent and additional rent payable hereunder, or either of them,
including, without limitation, any gross income tax or excise tax levied by the
City of Vacaville, the County of Solano, the State of California, the Federal
Government or any other governmental body with respect to the receipt of such
rent, but excluding any net income taxes;

      (c) Any and all taxes imposed upon or with respect to the possession,
leasing, operation, management, maintenance, alteration, repair, use or
occupancy by Tenant of the Premises or any portion thereof; and

      (d) Any and all taxes imposed upon this transaction or any document to
which Tenant is a party creating or transferring an interest or an estate in the
Premises. In the event that it shall not be lawful for Tenant to so reimburse
Landlord, the monthly rent payable to Landlord under this Lease shall be revised
to net Landlord the same net rent after imposition of any such tax upon Landlord
as would have been payable to Landlord prior to the imposition of any such tax.
All taxes payable by Tenant under this paragraph 26 shall be deemed to be, and
shall be paid as, additional rent.

                                   27. WAIVER

      The waiver by Landlord or Tenant of any breach of any agreement, covenant,
condition or provision herein contained shall not be deemed to be a waiver of
any subsequent breach of the same or any other agreement, covenant, condition or
provision herein contained, nor shall any custom or practice which may grow up
between Landlord and Tenant in the administration of this Lease be construed to
waive or to lessen the right of Landlord or Tenant to insist upon the
performance by Landlord or Tenant in strict accordance with this Lease. The
subsequent acceptance of rent hereunder by Landlord or the payment of rent by
Tenant shall not be deemed to be a waiver of any preceding breach by Landlord or
Tenant of any agreement, covenant, condition or provision of this Lease, other
than the failure of Tenant to pay the particular rent so accepted, regardless of
Landlord's or Tenant's knowledge of such preceding breach at the time of
acceptance or payment of such rent.


                                       14
<PAGE>   16
                                   28. NOTICES

      All notices and demands which may or are required to be given by either
Landlord or Tenant to the other hereunder shall be deemed to have been fully
given when made in writing and deposited in the United States mail, certified or
registered, postage prepaid, and addressed as follows: to Tenant at Tenant's
Address, or to such other place as Tenant may from time to time designate in a
notice to Landlord, or delivered to Tenant at the Premises; to Landlord at
Landlord's Address, or to such other place as Landlord may from time to time
designate in a notice to Tenant. Tenant hereby appoints as its agent to receive
the service of all dispossessory or distraint proceedings and notices thereunder
the person in charge of or occupying the Premises at the time and if no person
shall be in charge of or occupying the Premises, then such service may be made
by attaching the service on the main entrance of the Premises.

                             29. COMPLETE AGREEMENT

      There are no oral agreements between Landlord and Tenant affecting this
Lease, and this Lease supersedes and cancels any and all previous negotiations,
arrangements, brochures, letters of intent, agreements and understandings. There
are no representations between Landlord and Tenant or between any real estate
broker and Tenant other than those contained in this Lease. This Lease may not
be amended or modified in any respect whatsoever except by an instrument in
writing signed by Landlord and Tenant.

                                30. MISCELLANEOUS

      If there be more than one person or entity constituting the Tenant, the
obligations hereunder imposed upon Tenant shall be joint and several. Time is of
the essence of this Lease and each and all of its provisions. The agreements,
covenants, conditions and provisions herein contained shall, subject to the
provisions as to assignment, apply to and bind the heirs, executors,
administrators, successors and assigns of the parties hereto. If any provision
of this Lease shall be determined to be illegal or unenforceable, such
determination shall not affect any other provision of this Lease and all such
other provisions shall remain in full force and effect. This Lease shall be
governed by and construed in accordance with the local law of the State of
California, without regard to the principles of conflicts of law.

                             31. FINANCING CONDITION

      Landlord may obtain secured financing for the Building. If Landlord's
lender should require, as a condition to such financing, either execution by
Tenant of an agreement requiring Tenant to send such lender written notice of
any default by Landlord under this Lease, giving such lender the right to cure
such default until such lender has completed foreclosure and preventing Tenant
from terminating this Lease unless such default remains uncured after
foreclosure has been completed, or any modification of the agreements,
covenants, conditions, or provisions of this Lease, or both of them, then Tenant
agrees to execute and deliver such agreement and to modify the Lease as required
by such lender; provided, however, that no such modification shall affect the
length of the Term hereof or increase the rent payable by Tenant under
paragraphs 4 and 5 hereof. If Tenant shall fail or refuse to execute any such
agreement or


                                       15
<PAGE>   17
modification within ten (10) days after receipt thereof, Landlord shall have the
right, by giving written notice to Tenant, to terminate this Lease.

                             32. REAL ESTATE BROKERS

      Landlord and Tenant warrant and represent that Broker is the only broker
or real estate agent Tenant has used in conjunction with this transaction and
that Tenant has not authorized or employed, or acted by implication to authorize
or to employ, any other real estate broker or agent to act for Tenant in
connection with this Lease. Tenant shall hold Landlord harmless from and
indemnify and defend Landlord against any and all claims by any real estate
broker, agent or any other person, other than the Broker, for any commission,
finder's fee or other compensation as a result of Tenant's entering into this
Lease.

                             33. CORPORATE AUTHORITY

      If Tenant is a corporation, each person executing this Lease on behalf of
Tenant does hereby covenant and warrant that (a) Tenant is duly incorporated and
validly existing under the laws of its state of incorporation, (b) Tenant has
and is qualified to do business in California, (c) Tenant has full corporate
right and authority to enter into this Lease, and (d) each person signing this
Lease on behalf of the corporation is duly authorized to do so.

                                  34. GUARANTY

      Intentionally deleted.

                                 35. LATE CHARGE

      Tenant acknowledges that late payment by Tenant to Landlord of Rent,
additional rent or any other amount required to be paid under this Lease will
cause Landlord to incur costs not contemplated by this Lease, the exact amount
of which is extremely difficult and impracticable to ascertain. Such costs
include, without limitation, processing and accounting charges, and late charges
that may be imposed on Landlord by virtue of its debt obligations. Accordingly,
if Tenant falls to make any of such payments within ten (10) days after such
payment is due, Tenant shall pay an additional charge equal to the greater of
(i) interest on such payment, from the date due until paid at the lesser of (A)
the "prime rate" of interest announced by the Bank of America N.T. & S. A. as in
effect from day to day, plus five percent (5%) per annum, or (B) the maximum
rate permitted by law; or (ii) $100.00.

                            36. CONFLICT OF INTEREST

      Conflicts of interest relating to this Lease are strictly prohibited.
Except as otherwise expressly provided herein, neither Tenant nor any director,
employee or agent of Tenant shall give to or receive from any director, employee
or agent of Landlord any gift, entertainment or other favor of significant
value, or any commission, fee or rebate. Likewise, neither Tenant nor any
director, employee or agent of Tenant shall enter into any business relationship
with any director or employee of Landlord or of Landlord's wholly owned
subsidiaries, unless such person is acting for and on behalf of Landlord,
without prior written notification thereof to Landlord. An independent auditor
authorized by and paid for by Landlord may audit any and all


                                       16
<PAGE>   18
records of Tenant for the sole purpose of determining whether there has been
compliance with this paragraph.

                          37. TENANT REPRESENTATIONS

      If Tenant is a legal entity, Tenant hereby represents and warrants to
Landlord that (i) such entity is duly organized and validly existing under the
laws of the State of its formation and is qualified to do business in, and is in
good standing under, the laws of the State of California; and 00 this Lease and
all documents executed or to be executed by Tenant in connection herewith and
which are to be delivered to Landlord will be duly authorized, executed, and
delivered and will be legal, valid, and binding obligations of Tenant, and do
not violate any provisions of any agreement or currently existing judicial order
to which Tenant is a party or to which it is subject. Further, if requested by
Landlord, either prior to or after Landlord's execution of this Lease, Tenant
shall provide Landlord with certified evidence of the foregoing.

                                   38. RENEWAL

      Landlord grants to Tenant the option to extend the Term of this Lease for
a single period of five (5) years, at Landlord's then current published rental
rates and upon all other terms as detailed herein. Such option may only be
exercised by written notice thereof from Tenant to Landlord prior to the fourth
anniversary from the date hereof.

                               39. EXPANSION SPACE

      During the first five (5) years of the Term of this Lease and before
Landlord leases any or all of the space as shown on the attached Exhibit F,
contiguous to the Premises ("Expansion Space") to a third party, Landlord shall
advise Tenant of Landlord's intent to lease such space. Tenant shall have a
period of thirty (30) days from the date of such notice to advise Landlord if
Tenant desires to enter into a lease for the same space. If Tenant advises
Landlord that Tenant desires to lease the space, Landlord and Tenant shall
execute a lease with terms similar to those contained herein (other than those
terms contained in paragraphs 3, 39 and in Exhibit C) providing for a rental at
Landlord's then current published rental rate for the Building and a term of
years which shall terminate concurrently with this Lease. Such lease shall be
effective not later than the thirtieth day following Landlord's receipt of
Tenant's notification. If Tenant responds to Landlord's notice indicating that
Tenant is not interested in leasing such space or if Tenant falls to respond to
Landlord within such thirty (30) day period, Landlord shall not be obligated to
notify Tenant of a pending lease in the Expansion Space for a period of one
hundred eighty (180) days from the end of such thirty (30) day period.

                            40. ADDITIONAL PROVISIONS

      The Additional Provisions, if any, set forth in Exhibit H hereto are
hereby incorporated in this Lease. In the event of any conflict between the body
of this Lease and the Additional Provisions, the Additional Provisions shall
control.


                                       17
<PAGE>   19
                                  41. EXHIBITS

      The following exhibits, Exhibit A (Plan Outlining the Premises), Exhibit B
(Vaca Valley Business Park), Exhibit C (Work Agreement), Exhibit D (Rules and
Regulations), Exhibit E (Guaranty), Exhibit F (Expansion Space), Exhibit G
(Tenant's Trade Fixtures) and Exhibit H (Additional Provisions) are attached to
this Lease and by this reference made a part hereof.

                                   42. PARKING

      Landlord shall provide three (3) non-exclusive parking spaces (within the
Improved Area) for each 1,000 square feet of space (within the Premises) for the
use of Tenant, its employees and guests. The use of such parking spaces is
subject to the Rules and Regulations attached hereto.

      IN WITNESS WHEREOF, Landlord and Tenant have executed this Lease as of the
day and year first hereinabove written.

                                       LANDLORD:
                                       Sepulveda Properties, Inc., a
                                       California corporation


                                       By /s/ S.J. Alata
                                          --------------------------------------
                                          Title: President

                                       TENANT:
                                       Biosource, a California corporation


                                       By /s/ Carlos A. Santacruz
                                          --------------------------------------
                                          Its: Chief Financial Officer


                                       18
<PAGE>   20
                                    EXHIBIT A

                                    PREMISES



                                      A-1
<PAGE>   21

                          (Blue Print of the premises)



                                      A-2
<PAGE>   22
                                    EXHIBIT B

                            VACA VALLEY BUSINESS PARK

         (See Attached Map of Vaca Valley Business Park outlined in red)



                                      B-1
<PAGE>   23

                   (Location Map of Vaca Valley Business Park)



                                      B-2
<PAGE>   24
                                    EXHIBIT C

                                 WORK AGREEMENT

      The undersigned, as Landlord and Tenant, respectively, are executing
simultaneously with this Work Agreement, a written Lease covering premises as
described in the Lease in the building located at 3333 Vaca Valley Parkway,
Vacaville, California (the "Building"), and hereby attach this Work Agreement to
said Lease as Exhibit C thereto.

      Landlord and Tenant mutually agree as follows:

                      1. TENANT'S PLANS AND SPECIFICATIONS

            (a) Except to the extent otherwise provided in subparagraphs 1.(b)
and 1.(d) hereof, Landlord shall, at its sole cost and expense, furnish plans
and specifications required for the performance of the work (hereinafter
referred to as "Building Standard Work" and "Additional Improvements")
hereinbelow described.

            (b) Tenant may request work (hereinafter referred to as "Building
Non-standard Work") in lieu of or in addition to Building Standard Work. If
Landlord approves such request, any plans and specifications required for such
Building Non-standard Work shall be furnished, at Tenant's sole cost and
expense, by Landlord's architects and engineers. Landlord's approval shall not
be granted to change carpets, ceiling material, ceiling light fixtures, window
coverings and any other items which in Landlord's judgment would be detrimental
to the operation of the Building.

            (c) All plans and specifications referred to hereinabove in
subparagraphs (a) and (b) are subject to Landlord's approval.

            (d) Any changes to the Building's standard mechanical and electrical
systems requiring the approval of Landlord's engineers shall be subject to an
approval fee equal to the actual cost to Landlord of the services performed by
Its engineers.

            (e) When requested by Landlord's architects and engineers, Tenant
shall furnish complete information respecting Tenant's requirements for the
Premises. Complete plans, specifications and budgets shall be approved by Tenant
within ten (10) days after delivery to Tenant for approval.

                            2. BUILDING STANDARD WORK

            (a) BUILDING STANDARD WORK AT LANDLORD'S EXPENSE. Landlord has
established an allowance in the amount of $232,600.00 to provide Tenant with
office and laboratory space. The "Building Standard Work" which shall be
provided by Landlord up to such allowance includes:

      (a) Heating, ventilating and air conditioning system

      (b) Reasonable footage of interior partitions and doors


                                      C-1
<PAGE>   25
      (c) Wall covering

      (d) Acoustical ceilings

      (e) Ceiling lighting fixtures

      (f) Reasonable number of electric and telephone outlets

      (g) Carpeting

            (b) Additional Improvements. Landlord has established an allowance
in the amount of up to $300,000.00 to provide additional improvements to the
5,100 square foot laboratory port I on of the Premises, in consideration of such
improvements, Tenant agrees to pay to Landlord over the sixty (60) month Term of
this Lease, the sum of $6,321.00 per month in advance, as additional rent
hereunder. interest at the rate of ten percent (10%) per annum is included in
such sum.

Any Building Non-standard Work will be paid for by Tenant as provided in
paragraph 3 below.

           3. EXCESS BUILDING STANDARD AND BUILDING NOW STANDARD WORK

      Provided Tenant's plans and specifications are furnished by the date
provided hereinabove in subparagraph i(e) hereof, and approved by Landlord,
Landlord shall cause Tenant's "Building Non-standard Work" to be installed by
Landlord's contractor. Tenant shall bear all costs for Building Non-standard
Work. Prior to commencing any such work, Landlord shall submit to Tenant a
written estimate of the cost thereof. If Tenant shall fail to approve any such
estimate within ten (10) business days after submission thereof, such failure
shall be deemed a disapproval thereof, and Landlord's contractor shall not
proceed with such work. Tenant agrees to pay Landlord promptly upon being billed
for its share of the cost of all Building Standard Work and such Building
Non-standard Work. Landlord may require Tenant to deposit its portion of the
estimated cost of such work with Landlord prior to the commencement of such
work.

                   4. COMPLETION AND RENTAL COMMENCEMENT DATE

      When Landlord's architect has furnished Landlord with a certificate that
the work to be done by Landlord pursuant to paragraphs 2 and 3 above has been
substantially completed, the Premises will be deemed complete for all purposes
under the Lease. If Tenant has not yet occupied the Premises, Tenant's
obligation to pay rent shall not commence until Landlord's architect has
furnished such certificate; provided, however, that if Landlord shall be delayed
in substantially completing such work as a result of:

            (i)   Tenant's failure to furnish information so that plans and
                  specifications can be completed in accordance with the date
                  specified hereinabove in subparagraph 1.(e), or

            (ii)  Tenant's request for materials, finishes or installations
                  other than Landlord's "Building Standard Work," or


                                      C-2
<PAGE>   26
            (iii) Tenant's changes I n the plans and specifications approved by
                  him after their submission to Landlord in accordance with the
                  date specified in subparagraph 1.(e) hereinabove, or

            (iv)  Tenant's failure to approve the above estimates submitted by
                  Landlord to Tenant covering Building Non-standard Work,
                  pursuant to paragraph 3 above.

then the commencement of rent under said Lease shall be accelerated by the
number of days of such delay.

                                       LANDLORD:
                                       Sepulveda Properties, Inc., a
                                       California corporation


                                       By /s/  S. J. Alata
                                          --------------------------------------
                                          Its: President

                                       TENANT:
                                       Biosource, a California corporation


                                       By /s/  Carlos A. Santacruz
                                          --------------------------------------
                                          Its: Chief Financial Officer


      Landlord and Tenant hereby agree to the foregoing.


                                      C-3
<PAGE>   27
                                    EXHIBIT D

                              RULES AND REGULATIONS

      1. The sidewalks, halls, passages, exits, entrances, and stairways of the
Building shall not be obstructed by any of the tenants or used by them for any
purpose other than for ingress to and egress from their respective premises. The
halls, passages, exits, entrances and stairways are not for the general public
and Landlord shall in all cases retain the right to control and prevent access
thereto of all persons whose presence in the judgment of Landlord would be
prejudicial to the safety, character, reputation and interests of the Building
and its tenants, provided that nothing herein contained shall be construed to
prevent such access to persons with whom any tenant normally deals in the
ordinary course of its business, unless such persons are engaged in illegal
activities. No tenant and no employee, Invitee, or licensee of any tenant shall
go upon the roof of the Building. Landlord shall have the right any time without
the same constituting an actual or constructive eviction and without incurring
any liability to any tenant therefore to change the arrangement and/or location
of entrances or passageways, doors or doorways, corridors, stairs, toilets or
other common areas of the Building.

      2. No sign, placard, picture, name, advertisement or notice visible from
the exterior of any tenant's premises shall be inscribed, painted, affixed or
otherwise displayed by any tenant on any part of the Building without the prior
written consent of Landlord. Landlord shall adopt and furnish to tenants general
guidelines relating to signs inside the Building. Tenant agrees to conform to
such guidelines, as adopted. All approved signs or lettering on doors shall be
printed, painted, affixed or inscribed at the expense of the requesting tenant
by a person or company approved by Landlord. Material visible from outside the
Building will not be permitted.

      3. Neither the leased premises nor the Building shall be used for lodging.
No cooking shall be done or permitted in the Building except the private use by
a tenant of Underwriters' Laboratory approved equipment for brewing coffee, tea,
hot chocolate and similar beverages shall be permitted, provided that such use
is in accordance with all applicable Federal, state and municipal laws, codes
ordinances, rules and regulations.

      4. No tenant shall cause any unnecessary labor or expense by reason of
such tenant's carelessness or indifference in the preservation of good order and
cleanliness. Landlord shall not be responsible to any tenant for any loss of
property on or from the Building however occurring, or for any damage done to
the effects of any tenant by the janitor or any other employee or any other
person.

      5. Landlord shall furnish each tenant free of charge with two keys to each
door lock provided in the Premises by Landlord. Landlord may make a reasonable
charge for any additional keys. No tenant shall have any such keys copied or any
keys made. No tenant shall alter any lock or install a new or additional lock or
any bolt on any door of its premises. Each tenant, upon the termination of its
lease, shall deliver to Landlord all keys to doors in the Building.

      6. Landlord may designate an appropriate entrance for deliveries or other
movement to or from the Premises of equipment, materials, supplies, furniture or
other property, and no


                                      D-1
<PAGE>   28
tenant shall use any other entrances for such purposes. Landlord shall have the
right to prescribe the maximum weight, size and position of all equipment,
materials, furniture or other property brought into the Building. Heavy objects
shall, if considered necessary by Landlord, stand on a platform of such
thickness as is necessary to properly distribute the weight. Landlord shall not
be responsible for loss of or damage to any such property from any cause, and
all damage done to the Building by moving or maintaining such property shall be
repaired at the expense of Tenant.

      7. No tenant shall use or keep in Its premises or the Building any
inflammable or combustible material other than limited quantities thereof
reasonably necessary for the operation or maintenance of office equipment. No
tenant shall use any method of heating or air conditioning other than that
supplied by Landlord. No tenant shall use or keep any foul or noxious gas or
substance in its premises, or permit or suffer such premises to be occupied or
used in a manner offensive or objectionable to Landlord or other occupants of
the Building by reason of noise, odors or vibrations, or interfere in any way
with other tenants or those having business in the Building, nor shall any
animals or birds be brought or kept in any tenant's premises or the Building.

      8. Landlord shall have the right, exercisable without notice and without
liability to any tenant, to change the name or street address of the Building.

      9. Landlord reserves the right to exclude from the Building between the
hours of 7:00 P.M. and 7:00 A.M. and at all hours on Saturdays, Sundays and
legal holidays all persons who do not present identification acceptable to
Landlord. Each tenant shall provide Landlord with a list of all persons
authorized by such tenant to enter its premises and shall be liable to Landlord
for all acts of such persons. Landlord shall in no case be liable for damages
for any error with regard to the admission to or exclusion from the Building of
any person in the case of riot, public excitement or other circumstances
rendering such action advisable in Landlord's opinion, Landlord reserves the
right to prevent access to the Building during the continuance of the same by
such action as Landlord may deem appropriate, including closing doors.

      10. A directory of the Building shall be provided for the display of the
name and location of tenants. Landlord reserves the right to restrict the amount
of directory space utilized by any tenant.

      11. No curtains, draperies, blinds, shutters, shades, screens or other
coverings, hangings or decorations shall be attached to, hung or placed in, or
used in connection with any window of the Building without the prior written
consent of Landlord. In any event, with the prior written consent of Landlord,
such items shall be installed on the office side of Landlord's standard window
covering and shall in no way be visible from the exterior of the Building.

      12. Each tenant shall ensure that the doors of its premises are closed and
locked and that all water faucets, water apparatus and utilities are shut off
before such tenant or its employees leave such premises so as to prevent waste
or damage, and for any default or carelessness in this regard, each tenant shall
be liable for any and all injuries sustained by the other tenants or occupants
of the Building or Landlord. In the common area corridors, all tenants shall
keep the doors to the Building corridors closed at all times except for ingress
and egress.


                                      D-2
<PAGE>   29

      13. The toilet rooms, toilets, urinals, wash bowls and other apparatus
shall not be used for any purpose other than that for which they were
constructed. No foreign substance of any kind whatsoever shall be thrown therein
and the expense of any breakage, stoppage or damage resulting from the violation
of this rule shall be borne by the tenant who, or whose employees, invitees or
licensees thereof who have caused such problem.

      14. Except with the prior written consent of Landlord, no tenant shall
sell at retail newspapers, magazines, periodicals, theatre or travel tickets or
any other goods or merchandise to the general public in, on or from its
premises, nor shall any tenant carry on or permit or allow any employee or other
person to carry on the business of stenography, typewriting, printing or
photocopying or any similar business in or from its premises for the service or
accommodation of occupants of any other portion of the Building. No premises of
any tenant may be used for any business or activity other than that specifically
provided for in such tenant's lease.

      15. No tenant shall install any device on the roof or exterior walls of
the Building. No television, radio, recorder or other sound amplification device
shall be played in such a manner as to cause a nuisance to any other tenant.

      16. No other vehicles of any kind shall be brought into the Building.

      17. Each tenant shall store all its trash and garbage within its premises.
No material shall be placed in the trash boxes or receptacles if it may not be
disposed of in the ordinary and customary manner.

      18. Canvassing, soliciting, distribution of handbills or any other
written material, and peddling in the Building are prohibited.

      19. The requirements of tenants shall be attended to only upon application
in writing to Landlord. Employees of Landlord shall not perform any work or do
anything outside of their regular duties unless under special instructions from
Landlord.

      20. Landlord may waive any of these Rules and Regulations for the benefit
of any particular tenant, but no such waiver by Landlord shall be construed as a
waiver of such Rules and Regulations in favor of any other tenant nor prevent
Landlord from thereafter enforcing any such Rules and Regulations against any or
all of the tenants of the Building.

      21. These Rules and Regulations are in addition to, and shall not be
construed to in any way modify or amend, in whole or In part, the agreements,
covenants, conditions and provisions of any lease of premises in the Building.

      22. Landlord reserves the right to amend these Rules and Regulations or to
make such other rules and regulations as in its judgment may from time to time
be needed for the safety, care and cleanliness of the Building and for the
preservation of good order therein.


                                      D-3
<PAGE>   30
                                    EXHIBIT E

                                    GUARANTY

                            Intentionally left blank.



                                      E-1
<PAGE>   31
                                    EXHIBIT F

                                 EXPANSION SPACE

                               (see attached plan)



                                      F-1
<PAGE>   32

                         (Blue Print of Expansion Space)



                                      F-2
<PAGE>   33
                                    EXHIBIT G

                             TENANT'S TRADE FIXTURES

Landlord and Tenant acknowledge that during the Term of this Lease, Tenant may
need to use certain personal property and to install certain fixtures in the
Premises which are unique to Tenant's business, the installation of some of
which may require physical attachment to the Premises, but which may be removed
without damaging the Premises ("Trade Fixtures"). In the event Landlord supplies
any personal property or Trade Fixtures to Tenant for Tenant's use in the
Premises, such personal property and Trade Fixtures shall become the property of
Tenant upon the expiration of the Term of this Lease as stated herein, provided
Tenant is not then in default of any of the terms of this Lease. All personal
property and Trade Fixtures not becoming the property of Tenant as provided
above shall be returned to Landlord upon the expiration or termination of this
Lease in good condition (subject to reasonable wear and tear) and as otherwise
provided in Paragraph 10 of this Lease. Any personal property and Trade Fixtures
provided by Landlord and the Trade Fixtures supplied by Tenant are listed below.

Those Trade Fixtures supplied by Tenant as listed below shall remain the
property of Tenant upon the expiration or termination of this Lease.

These lists may be modified at any time and from time to time by the mutual
written agreement of the parties hereto.


Personal Property and Trade Fixtures supplied by Landlord:



Trade Fixtures Supplied by Tenant:


                                      G-1
<PAGE>   34

                  FIRST AMENDMENT TO VACA VALLEY BUSINESS PARK
                                  OFFICE LEASE


I.      Parties

        The parties to this First Amendment to Vaca Valley Business Park Office
Lease ("Amendment") are Mission -Messenger Vacaville General Partnership, a
California general partnership ("Landlord"), successor-in-interest to Sepulveda
Properties, Inc., a California corporation ("Original Landlord"), and Biosource
Genetics Corporation, a California corporation ("Tenant").

II.     Recitals

        A. On or about October 15, 1987, Original Landlord entered into that
certain Vaca Valley Business Park Office Lease ("Lease") for a building located
on certain property at 3333 Vaca Valley Parkway, Vacaville, California.
Subsequent thereto, Landlord acquired Original Landlord's interest in the Lease.

        B. The parties deem it to be in their mutual best interest to amend the
Lease as set forth herein.

        C. Except as otherwise provided in this Amendment, all capitalized terms
as used herein shall have the same meaning as in the Lease.

III.    Amendment

        For valuable consideration, the receipt and sufficiency of which is
hereby acknowledged, and the mutual covenants and promises contained herein, the
parties agree as follows:

        A. Premises. The first three lines of the definition of "Premises" in
Section I of the Lease entitled "Lease Summary" are deleted in its entirety and
the following is inserted in its place and stead: "The interior space shown
outlined in red on the floor plan of the Building attached hereto as Exhibit "A"
and incorporated herein by reference (containing approximately 12,583 square
feet); including, the appurtenant right to use, in common with others,
the . . ."

        B. Base Rent. The Base Rent as defined on page 2 of the Lease, shall be
$10,808.80 per month beginning on July 1, 1989. The Base Rent shall continue to
be subject to adjustment according to the provisions of Section 4 of the Lease.

        C. Tenant's Share. Tenant's Share, as defined on page 2 of the Lease,
shall be 26.6% of the Improved Area.

        D. Tenant Improvements. The Lease shall be amended and supplemented by
the addition of Section 43 as follows:

            "43. Tenant Improvement Allowance.


                                First Amendment
<PAGE>   35

            (a) Landlord shall construct, install, and pay an amount not to
exceed $65,580 for standard tenant improvements ("Tenant Improvement
Allowance"). The Tenant Improvement Allowance shall be used for the preparation
by Landlord's architect and/or space planner for plans and working drawings for
the tenant improvements described on Exhibit "B" attached hereto and
incorporated herein by reference ("Tenant Improvements") and for construction of
the Tenant Improvements by a general contractor mutually acceptable to Landlord
and Tenant. No portion of the Tenant Improvement Allowance may be used for
furnishings and/or trade fixtures or any item that may be removable by Tenant
upon the expiration of the term of the Lease. Landlord shall deliver to Tenant
cost estimates of any proposed Tenant Improvements. If Tenant has not objected
to such cost estimates within ten (10) days of receipt of such cost estimates,
such cost estimates shall be deemed approved. In the event that Tenant approves
or is deemed to have approved a cost estimate in an amount less than the Tenant
Improvement Allowance, Tenant shall not be entitled to any credit or reduction
in Basic Rent. If Tenant disapproves the estimate Landlord shall not proceed
with the Tenant Improvements, but Landlord and Tenant shall thereafter cooperate
to amend the plans and specifications for the Premises as necessary to obtain
Tenant's approval for the costs thereof.

            (b) Landlord shall construct, install, and pay for the cost of
non-standard tenant improvements (included in Exhibit "B" attached hereto and
incorporated herein by reference) up to an amount of $140,000 ("Additional
Cost"); provided, however, that Tenant shall reimburse Landlord for such
Additional Cost, in equal monthly installments, by an amount sufficient to
return to Landlord the Additional Cost amortized over the remaining term of the
Lease and interest on the outstanding principal balance of the Additional Cost
at 11-1/2% per year. (By way of example only, if the Additional Cost is $100,000
and the remaining term of the Lease is thirty-six (36) months, the monthly
installments for the Additional Cost owed by Tenant would be $3,297.60.)
Notwithstanding any provision herein, the Additional Cost and all accrued
interest thereon is immediately due and payable upon the termination or
expiration of the Lease. Any late payment of Additional Cost by Tenant shall be
subject to Section 35 of the Lease (entitled "Late Charge"). Any amount in
excess of the $140,000 shall be paid by Tenant within ten (10) days of receipt
of a bill for such amount. At Landlord's option, work on Tenant Improvements may
be halted pending the curing of any default under the Lease or Amendment.

            (c) By July 1, 1989, Landlord shall construct or install the
improvements shown in Exhibit B on the Premises in accordance with Tenant's
specifications and specifications of Landlord's architect.

            (d) If the improvements set forth in Exhibit B are not substantially
completed by July 1, 1989, or if Landlord for any reason whatsoever cannot
deliver possession of the portion of the Premises covered by Exhibit B by July
1, 1989, then all increases in Rent including the Expense Stop, Operating
Expenses and reimbursements of Additional Costs required by subsections B, C,
and D of Section III of this Amendment shall be waived until such time as
Landlord can deliver possession."

        E. Governing Provisions. In all other respects, the Lease shall remain
unchanged and in full force and effect. If there is a conflict between the terms
and conditions of this Amendment and the terms and conditions of the Lease, the
terms and conditions of this Amendment shall govern.


                                First Amendment-2
<PAGE>   36

        F. Attorneys' Fees. Should any party retain counsel to enforce any of
the provisions of this Amendment or protect its interest in any matter arising
hereunder, the losing party in any action shall pay to the prevailing party all
costs, damages, and expenses incurred by the prevailing party, including without
limitation, attorneys' fees and costs incurred in connection therewith.

            This Amendment is executed as of the day and year first above
written.

        This Amendment is executed as of the day and year first above written.

Landlord:

Mission-Messenger Vacaville
General Partnership

By:     Messenger Partnership XIV, L.P., a California Limited Partnership

        By:     Messenger Investment Company, a California corporation, General
                Partner


                         By:   /s/ William S. Messenger
                               -------------------------------------------
                         Its:  President
                               -------------------------------------------

        By:              Mission Land Company, a California Corporation
                         By:   /s/ Robert E. Umbaugh
                               -------------------------------------------
                         Its:  President
                               -------------------------------------------


Biosource Genetics Corporation, A California corporation

By:   /s/ David R. Mc Gee
      -------------------------------------------
Its:  Chief Operating Officer
      -------------------------------------------


                                First Amendment-3
<PAGE>   37

                                    EXHIBIT A


                                   Exhibit A-1
<PAGE>   38

                           (Overview of Parkway Plaza)


                                   Exhibit A-2
<PAGE>   39

                  SECOND AMENDMENT TO VACA VALLEY BUSINESS PARK
                                  OFFICE LEASE


I.      Parties

        The parties to this Second Amendment dated January 31, 1991 to Vaca
Valley Business Park Office Lease ("Amendment") are Mission-Messenger Vacaville
General Partnership, a California general partnership ("Landlord"),
successor-in-interest to Sepulveda Properties, Inc., a California corporation
("Original Landlord"), and Biosource Genetics Corporation, a California
corporation ("Tenant").

II.     Recitals

        A. On or about October 15, 1987, Original Landlord entered into that
certain Vaca Valley Business Park Office Lease ("Lease") for a building located
on certain property at 3333 Vaca Valley Parkway, Vacaville, California.
Subsequent thereto, Landlord acquired Original Landlord's interest in the Lease,

        B. On or about July 1, 1989 the parties entered into the First
Amendment to Vaca Valley Business Park Office Lease.

        C. The parties deem it to be in their mutual best interest to amend
the Lease as set forth herein.

        D. Except as otherwise provided in this Amendment, all capitalized
terms as used herein shall have the same meaning as in the Lease.

III.    Amendment

        For valuable consideration, the receipt and sufficiency of which is
hereby acknowledged, and the mutual covenants and promises contained herein, the
parties agree as follows:

        A. Premises. The first three lines as amended of the definition of
"Premises" in Section 1 of the Lease entitled "Lease Summary" are deleted in its
entirety and the following is inserted in its place and stead: "The interior
space shown outlined in red on the floor plan of the building attached hereto as
Exhibit "A" and incorporated herein by reference (containing approximately
15,367 square feet); including the appurtenant right to use, in common with
others, the ..."

        B. Base Rent. The Base Rent as defined on page 2 of the Lease, shall
be $12,902.53 per month beginning on March 15,1992 or as otherwise dictated by
section Ill paragraph D, subparagraphs (b) and (c) of this amendment. The Base
Rent shall continue to be subject to adjustment according to the provisions of
Section 4 of the Lease.

        C. Additional Rent: In addition to Base Rent, there shall be
Additional Rent of $130.84 per month beginning on March 15, 1992 and extending
through December 15, 1992,


                                Second Amendment
<PAGE>   40

        D. Tenant's Share. Tenant's Share, as defined on page 2 of the
Lease, shall be 32.32% of the Improved Area.

        E. Tenant Improvements.

           (a) Landlord shall construct, install, and pay for tenant
improvements. The Tenant Improvements shall include the preparation by
Landlord's architect and/or space planner for plans and working drawings for the
tenant improvements described on Exhibit "B" attached hereto and incorporated
herein by reference ("Tenant Improvements") and permits and construction of the
Tenant Improvements by a contractor mutually acceptable to Landlord and Tenant.
Landlord shall deliver to Tenant for approval the plans for any proposed Tenant
Improvements. If Tenant has not objected to such plans within five (5) days of
receipt of such, said plans shall be deemed approved. If Tenant disapproves the
plans, Landlord and Tenant shall thereafter cooperate to amend the plans and
specifications for the Premises as necessary to obtain Tenant's approval
thereof.

           (b) Provided tenant has finally approved the plans and
specifications by no later than February 15, 1992, Landlord shall construct or
install the improvements shown in Exhibit B on the Premises on or before March
15, 1992.

           (c) If the improvements set forth in Exhibit B are not substantially
completed by March 15, 1992, or if Landlord for any reason whatsoever cannot
deliver possession of the Premises covered by Exhibit B by March 15, 1992, then
all increases in Rent including the Expense Stop and Operating Expenses required
by subsections B and C of Section III of this Amendment shall be waived until
such time as Landlord can deliver possession.

        F. Governing Provisions. In all other respects, the Lease shall remain
unchanged and in full force and effect. If there is a conflict between the terms
and conditions of this Amendment and the terms and conditions of the Lease, the
terms and conditions of this Amendment shall govern.

        G. Attorneys' Fees. Should any party retain counsel to enforce any of
the provisions of this Amendment or protect its interest in any matter arising
hereunder, the losing party in any action shall pay to the prevailing party all
costs, damages, and expenses incurred by the prevailing party, including without
limitation, attorneys' fees and costs incurred in connection therewith.

        This Amendment is executed as of the day and year first above written.

Landlord:

Mission-Messenger Vacaville
General Partnership

By:     Messenger Partnership XIV, L.P., a California Limited Partnership

        By:     Messenger Investment Company, a California corporation, General
                Partner


                               Second Amendment-2
<PAGE>   41

                         By:   /s/ William S. Messenger
                               -------------------------------------------

                         Its:
                               -------------------------------------------
                                              President
Tenant

Biosource Genetics Corporation, A California corporation

By:   /s/ David R. McGee
      -------------------------------------------
Its:      Vice President
      -------------------------------------------


                               Second Amendment-3
<PAGE>   42

                                    EXHIBIT A


                                   Exhibit A-1
<PAGE>   43

                 (Visual Schematic of Interchange Business Park)


                                   Exhibit A-2
<PAGE>   44

                                    EXHIBIT B


1.      Plans and working drawings.

2.      Building permits.

3.      Re-key two (2) existing door locks to the sub-master lock system
        assigned to Biosource.

4.      Install two (2) illuminated exit signs over existing doors.

5.      Install four (4) chain hung single tube fluorescent light fixtures. Said
        lights to be powered from the existing Biosource electric meter.

6.      Install wall mounted switching for lighting at each exit door.

7.      Install one (1) duplex wall outlet to be located on the existing north
        partition walls. Said outlet to be powered from the existing Biosource
        electric meter.

8.      Install one (1) fire alarm pull station if so required by the Vacaville
        Fire Department.

9.      Install ceiling insulation.

10.     Install one (1) set of double doors on the north wall of the expansion
        space, providing access to the existing space.

11.     Provide full height demising wall on the north and west side of the
        leased space.

12.     Provide one recessed fire extinguisher box.


                                   Exhibit B-1
<PAGE>   45

            THIRD AMENDMENT TO VACA VALLEY BUSINESS PARK OFFICE LEASE

             BETWEEN MISSION-MESSENGER VACAVILLE GENERAL PARTNERSHIP

                       AND BIOSOURCE GENETICS CORPORATION

I.      Parties

        The parties to this Third Amendment dated June 30,1992 to Vaca Valley
Business Park Office Lease ("Amendment") are Mission-Messenger Vacaville General
Partnership, a California general partnership ("Landlord"),
successor-in-interest to Sepulveda Properties, Inc., a California corporation
("Original Landlord"), and Biosource Genetics Corporation, a California
corporation ("Tenant").

II.     Recitals

        A. On or about October 15, 1987, Original Landlord entered into that
certain Vaca Valley Business Park Office Lease ("Lease") for a building located
on certain property at 3333 Vaca Valley Parkway, Vacaville, California.
Subsequent thereto, Landlord `acquired Original Landlord's interest in the
Lease.

        B. On or about July 1, 1989 the parties entered into the First
Amendment to Vaca Valley Business Park Office Lease.

        C. On or about January 31, 1992 the parties entered into the Second
Amendment to Vaca Valley Business Park Office Lease.

        D. The parties deem it to be in their mutual best interest to amend
the Lease as set forth herein.

        E. Except as otherwise provided in this Amendment, all capitalized
terms as used herein shall have the same meaning as in the Lease, as amended.
References to the Lease herein shall mean the Lease as previously amended.

III.    Amendment

        For valuable consideration, the receipt and sufficiency of which is
hereby acknowledged, and the mutual covenants and promises contained herein, the
parties agree as follows:

        A. Base Rent: Notwithstanding anything contained in the Lease to the
contrary, the Base Rent as defined on page 2 of the Lease shall be $12,293.60
per month beginning January 1, 1993 and shall remain the same for the balance of
the term of the Lease.

        B. Abatement: Base Rent for the period commencing July 1, 1992 and
continuing through December 31, 1992 shall be abated, subject to Tenant's timely
payment and performance of all its other obligations under the Lease.

        C. Operating Costs Adjustments: The first sentence of Section 5(a) of
the Lease entitled "Operating Costs Adjustments" is hereby deleted in its
entirety and the following is


                                Third Amendment
<PAGE>   46

inserted in its place and stead: "Tenant shall pay Tenant's Share of "Operating
Expenses" of $0.27/SF with no escalations; to the extent that actual Tenant's
Share exceeds $0.27/SF, Tenant will not be responsible for excess Operating
Expenses; to the extent that actual Tenant's Share of Operating Expenses are
less than $0.27/SF, Landlord will accrue such excess balance into a building
reserve fund; if at the termination of the lease funds are still available in
the building reserve fund, said excess will be reimbursed to the Tenant."

        Paragraph (b) and (c) in Section 5 entitled "Operating Costs
Adjustments" is deleted in its entirety.

        D. Term: The Term of the Lease is hereby extended for five (5)
additional years, and is to expire on the Expiration Date set forth below.

        E. Expiration Date: The Expiration Date as defined on page 2 of the
Lease shall be December 31, 1997.

        F. Early Termination: Provided Tenant is not then in default
hereunder, Tenant shall be granted a right to terminate the Lease subject to the
following conditions:

        (1) Tenant shall provide to Landlord not less than the number of full
calendar months prior written notice ("Termination Notice") applicable with
respect to such termination as specified on Exhibit "A" attached hereto;
provided that such notice shall not be served upon Landlord prior to July 1,
1995.

        (2) Tenant shall pay to Landlord upon delivery of said written
Termination Notice the equivalent of six (6) months Base Rent hereunder (i.e.,
$73,761.60) as compensation for such early termination of the lease;

        (3) In the event that the Termination Date would occur after June 30,
1997, pursuant to the above notification, Tenant shall pay to Landlord an amount
equal to the Monthly Base Rent multiplied by the number of full or partial
months remaining in the Term as compensation for the early termination of the
Lease, as specified on Exhibit "A" attached hereto;

        (4) Attached as Exhibit "A", and incorporated into this Amendment, is
a table evidencing the required notification period and the applicable
termination fee for any early termination hereunder.

        G. Governing Provisions: In all other respects, the Lease shall remain
unchanged and in full force and affect. If there is a conflict between the terms
and conditions of this Amendment and the terms and conditions of the Lease, the
terms and conditions of this Amendment shall govern.

        H. Attorney's Fees: Should any party retain counsel to enforce any of
the provisions of this Amendment or protect its interest in any matter arising
hereunder, the losing party in any action shall pay to the prevailing party all
costs, damages, and expenses incurred by the prevailing party, including without
limitation, attorneys' fees and costs incurred in connection therewith.



                                Third Amendment-2
<PAGE>   47
        This Amendment is executed as of the day and year first above written.

Landlord:

Mission-Messenger Vacaville
General Partnership, a California General Partnership

By:     Messenger Partnership XIV, L.P., a California Limited Partnership,
        General Partner

        By:     Messenger Investment Company, a California corporation, General
                Partner


                               By:   /s/ William S. Messenger, Jr.
                                     -------------------------------------------
                                              William S. Messenger, Jr.
                               Its:
                                     -------------------------------------------
                                                      President

Tenant

Biosource Genetics Corporation, A California corporation

                 By:   /s/ Kevin J. Ryan
                       -------------------------------------------
                                     Kevin J. Ryan
                 Its:
                       -------------------------------------------
                           President and Chief Operating Officer


                                Third Amendment-3
<PAGE>   48

                                    EXHIBIT A


                  BIOSOURCE GENETICS EARLY TERMINATION SCHEDULE
<TABLE>
<CAPTION>
                                                                                  APPLICABLE TERMINATION
   MONTH DURING WHICH TERMINATION           REQUIRED NOTIFICATION                  FEE (NUMBER OF MONTHS
       NOTICE IS TO BE GIVEN                     (IN MONTHS)                      OF BASE RENT TO BE PAID)

<S>                                   <C>                                         <C>
         Prior to June 1995           *** No early termination option ***
     July 1995 - September 1996                      9                                    6
            October 1996                             9                                    6
           November 1996                             8                                    6
           December 1996                             7                                    6
            January 1997                             6                                    6
           February 1997                             5                                    6
             March 1997                              4                                    6
             April 1997                              3                                    6
              May 1997                               2                                    6
             June 1997                               1                                    6
             July 1997                               1                                    5
            August 1997                              1                                    4
           September 1997                            1                                    3
            October 1997                             1                                    2
           November 1997                             1                                    1
           December 1997                             1                                    0
</TABLE>


                                   Exhibit A-1
<PAGE>   49

                            FOURTH AMENDMENT TO LEASE


        THIS FOURTH AMENDMENT TO LEASE (this "AMENDMENT") is made and entered
into as of November 22, 1995, by and between MISSION VACAVILLE LIMITED
PARTNERSHIP, a California limited partnership ("LANDLORD"), and BIOSOURCE
GENETICS CORPORATION, a California corporation ("TENANT"), with reference to the
following Recitals:

                                   BACKGROUND

            A. Landlord (as successor in interest to Sepulveda Properties, Inc.)
        and Tenant are currently parties to the Vaca Valley Business Park Office
        Lease dated as of October 15, 1987, as amended by the First Amendment
        thereto dated as of July 1, 1989, the Second Amendment thereto dated as
        of January 1, 1992, and the Third Amendment thereto dated as of June 30,
        1992 (as amended, the "LEASE"), pursuant to which Tenant currently
        leases certain office space within the Interchange Business Park -
        Vacaville, at 3333 Vaca Valley Parkway, Vacaville, California 95688,
        which space is more particularly described in the Lease (the
        "PREMISES").

            B. Tenant now desires to expand the Premises by leasing from
        Landlord approximately 3,400 additional square feet in the Building
        currently identified as Suite 700 and more particularly described on the
        floor plan attached hereto as Exhibit A (the "EXPANSION SPACE").
        Landlord has agreed to lease the Expansion Space to Tenant on the terms
        and conditions of the Lease and this Second Amendment.

            NOW, THEREFORE, in consideration of the foregoing Recitals, the
agreements set forth herein, and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, Landlord and Tenant
hereby agree as follows:

        1. DEFINITIONS. All initially capitalized terms not otherwise defined
herein shall have the meanings set forth in the Lease. From and after the date
hereof, references to the Lease shall mean the Lease as amended hereby. In the
event of any inconsistency between the terms of the Lease as amended to date and
the terms of this Amendment, the terms of this Amendment shall prevail.

        2. EXPANSION OF PREMISES. Landlord hereby leases the Expansion Space to
Tenant on the terms set forth herein, and Tenant hereby agrees to lease the
Expansion Space from Landlord. From and after January 15, 1996 (the "EXPANSION
DATE"), the Expansion Space shall become a part of the Premises for all purposes
under the Lease. Tenant acknowledges that Tenant has inspected the Expansion
Space, is familiar with the condition thereof and hereby accepts the Expansion
Space in an "AS IS" physical condition and in an "AS IS" state of repair. Tenant
further acknowledges that Landlord is not obligated to construct, install, or
pay for any improvements of any kind whatsoever to the Expansion Space to
prepare it for Tenant's occupancy.

        3. RENT. The Base Rent payable by Tenant pursuant to Section 4 of the
Lease is increased to $15,013.60 per month effective as of the Expansion Date.
The Base Rent shall


                                Fourth Amendment
<PAGE>   50

continue to be subject to adjustment according to the provisions of Section 4 of
the Lease.

        4. TENANT'S SHARE. The Tenant's Share under the Lease is increased to
39.48% effective as of the Expansion Date.

        5. RATIFICATION. The Lease, as amended by this Amendment, is hereby
ratified and confirmed in all respects and remains in full force and effect.
This Amendment shall be effective as of the date hereof upon execution and
delivery by both Landlord and Tenant. Except for those provisions of the Lease
which are expressly amended hereby, the Lease remains unmodified and unchanged.

                         [SIGNATURES ON FOLLOWING PAGE]


                               Fourth Amendment-2
<PAGE>   51

        IN WITNESS WHEREOF, Tenant and Landlord's duly authorized
representatives have executed this Amendment as of the date first above written.

                                    "LANDLORD"

                                    MISSION VACAVILLE LIMITED PARTNERSHIP,
                                    a California limited partnership

                                    By:     Mission Land Company,
                                            Its General Partner

                                            By: /s/ Sunny Lee
                                                ----------------------------
                                            Name:
                                                ----------------------------
                                            Its:
                                                ----------------------------

                                    "TENANT"

                                    BIOSOURCE TECHNOLOGIES, INC.,
                                    a California corporation

                                    By: /s/ David R. McGee
                                        --------------------------------
                                    Name:
                                         -------------------------------
                                    Its:
                                         -------------------------------


                               Fourth Amendment-3
<PAGE>   52

           FORM OF MEMORANDUM OF EXPANSION PREMISES COMMENCEMENT DATE

This Memorandum of Additional Premises Commencement Date is made as of December
15, 1995 by MISSION VACAVILLE LIMITED PARTNERSHIP, a California limited
partnership ("Landlord") and BIOSOURCE TECHNOLOGIES, INC., a California
corporation ("Tenant"). Landlord and Tenant hereby agree to and acknowledge the
following matters:

        1.      Landlord and Tenant have entered into that certain Fourth
                Amendment To Lease dated as of November 22, 1995 (the "Lease
                Amendment"), covering certain office space in the Building
                located at 3333 Vaca Valley Parkway, Vacaville, California
                95688, as more particularly described in the Lease Amendment.

        2.      All terms defined or referenced in the Lease Amendment shall
                have the same meaning when used herein.

        3.      Landlord and Tenant hereby confirm and agree that the expansion
                space Commencement Date is December 26, 1995.

IN WITNESS WHEREOF, Landlord and Tenant have executed this Memorandum of
Expansion Premises Commencement Date as of the date first above written.

                                "LANDLORD"

                                MISSION VACAVILLE LIMITED PARTNERSHIP,
                                a California limited partnership

                                By:   Mission Land Company, It General Partner

                                      By: /s/ Sunny Lee
                                          ----------------------------
                                      Name:
                                           ---------------------------
                                      Its:
                                          ----------------------------


                                "TENANT"

                                BIOSOURCE TECHNOLOGIES, INC.,
                                a California corporation

                                By: /s/ David R. McGee
                                    --------------------------------
                                Name:
                                     -------------------------------
                                Its:
                                    --------------------------------


                               Form of Memorandum
<PAGE>   53

                            FIFTH AMENDMENT TO LEASE

        THIS FIFTH AMENDMENT TO LEASE (this "AMENDMENT") is made and entered
into as of November 22,1996, by and between MISSION VACAVILLE LIMITED
PARTNERSHIP, a California limited partnership ("LANDLORD"), and BIOSOURCE
GENETICS CORPORATION, a California corporation ("TENANT"), with reference to the
following Recitals:

                                   BACKGROUND

            A. Landlord (as successor in interest to Sepulveda Properties, Inc.)
and Tenant are currently parties to the Vaca Valley Business Park Office Lease
dated as of October 15, 1987, as amended by the First Amendment thereto dated as
of July 1, 1989, the Second Amendment thereto dated as of January 1, 1992, the
Third Amendment thereto dated as of June 30, 1992 and the Fourth Amendment
thereto dated as of November 22, 1995 (as amended, the "Lease"), pursuant to
which Tenant currently leases certain office space within the Interchange
Business Park - Vacaville, at 3333 Vaca Valley Parkway, Vacaville, California
95688, which space is more particularly described in the Lease (the "PREMISES").

            B. Tenant now desires to extend the term of the Lease and amend its
terms in the manner and on the terms and conditions set forth in this Fifth
Amendment.

            NOW, THEREFORE, in consideration of the foregoing Recitals, the
agreements set forth herein, and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, Landlord and Tenant
hereby agree as follows:

        1. DEFINITIONS. All initially capitalized terms not otherwise defined
herein shall have the meanings set forth in the Lease. From and after the date
hereof, references to the Lease shall mean the Lease as amended hereby. In the
event of any inconsistency between the terms of the Lease as amended to date and
the terms of this Amendment, the terms of this Amendment shall prevail.

        2. EXTENSION OF LEASE TERM. The Term of the Lease is hereby extended by
three (3) additional years, such that the Expiration Date of the Lease (as
originally defined on page 2 of the Lease and as extended pursuant to Paragraph
III(D) of the Third Amendment to the Lease) is now December 31, 2000.

        3. OPTION TO EXTEND THE LEASE TERM.

            (a) Provided Tenant has complied with all of the terms and
conditions of this Lease, Tenant shall have the option to extend the term of
this Lease for a period of three (3) years (the "Extension Term"), commencing on
January 1, 2001, and ending on December 31, 2003, on the same terms and
conditions as those contained in the Lease, except that the Base Rent during the
Extension Term shall be Fair Market Rent (defined below) subject to annual
adjustment on the terms set forth in subsection (e) below. In addition to the
Base Rent to be paid by Tenant during each Extension Term, Tenant shall continue
to pay all additional charges under the terms of the Lease, including additional
rent with respect to Operating Expenses over the Expense Stop.


                                Fifth Amendment
<PAGE>   54

            (b) To exercise the Extension Option, Tenant must give written
notice to Landlord no later than twelve (12) months prior to the Expiration
Date. Any exercise notice given more than fifteen (15) months prior to the
Expiration Date is not effective. Tenant may not exercise the Extension Option
if Tenant is in default under any of the terms and conditions of the Lease, but
Landlord may waive this restriction in its sole discretion. Tenant has not
further options to extend the Lease.

            (c) The term "Fair Market Rental" means the rent that a willing
landlord would agree to accept from a willing, non-renewal tenant, in an
arms-length negotiation, for a three (3) year lease of the Premises commencing
as of the Expiration Date, taking into account the quality of the improvements,
the terms of the Lease, the allocation of costs thereunder, and all other
relevant factors. Landlord shall not be required, in connection with the
determination of the Base Rent, to amend the terms of the Lease, to grant free
rent or any other inducements or concessions, to construct or pay for new or
additional tenant improvements or to perform any work at the Premises (and the
foregoing shall be taken into consideration in determining the Fair Market
Rental).

            (d) Within thirty (30) days after receiving Tenant's notice of its
intent to extend the Lease, Landlord shall deliver to Tenant a written estimate
(the "Rent Notice") of Fair Market Rental for the Premises. If Landlord falls to
deliver such notice by the Rent Notice Date, Tenant shall demand such notice in
writing, and Landlord shall deliver a Rent Notice to Tenant within 20 days after
receipt of such written demand. If Tenant does not accept Landlord's estimate of
the Fair Market Rental, then for a period of 30 days following Tenant's receipt
of such notice, Landlord and Tenant shall reasonable endeavor, through good
faith negotiations, to mutually agree upon the Fair Market Rental. If Landlord
and Tenant are unable to agree upon the Fair Market Renal within such 30- day
period, then either Landlord or Tenant may terminate the negotiations, in which
event the Lease shall expire on the Expiration Date, and Tenant shall have no
further option to extend the Lease.

            (e) If Tenant exercises the Extension Option, the Base Rent shall be
subject to adjustment increase in proportion to the increase in the CPI-U, in
the following manner:

                 (1) On January 1, 2001, the Base Rent shall be multiplied by
the quotient of (A) the CPI-U published most recently prior to such date divided
by (B) the CPI-U published most recently as of January 1, 2000, provided that
the increase shall not be less than four percent (4%) and shall not be more than
eight percent (8%); and

                 (2) On January 1, 2002, the Base Rent shall be multiplied by
the quotient of (A) the CPI-U published most recently prior to such date divided
by (B) the CPI-U published most recently as of January 1, 20001, provided that
the increase shall not be less than four percent (4%) and shall not be more than
eight percent (8%).


                               Fifth Amendment-2
<PAGE>   55

        4. RATIFICATION. The Lease, as amended by this Amendment, is hereby
ratified and confirmed in all respects and remains in full force and effect.
This Amendment shall be effective as of the date hereof upon execution and
delivery by both Landlord and Tenant. Except for those provisions of the Lease
which are expressly amended hereby, the Lease remains unmodified and unchanged.

                         [SIGNATURES ON FOLLOWING PAGE]


                               Fifth Amendment-3
<PAGE>   56

        IN WITNESS WHEREOF, Tenant and Landlord have executed this Amendment as
of the date first above written.

                                    "LANDLORD"

                                    MISSION VACAVILLE LIMITED PARTNERSHIP,
                                    a California limited partnership

                                    By:     Mission Land Company,
                                            Its General Partner

                                            By:/s/ Sunny Lee
                                               -----------------------------
                                            Name:
                                                 ---------------------------
                                            Its:
                                                 ---------------------------

                                    "TENANT"

                                    BIOSOURCE TECHNOLOGIES, INC.,
                                    a California corporation

                                    By: /s/ Bruce A. Boyd
                                        --------------------------------
                                    Name:
                                         -------------------------------
                                    Its:
                                         -------------------------------


                               Fifth Amendment-4
<PAGE>   57

                            SIXTH AMENDMENT TO LEASE

        THIS SIXTH AMENDMENT TO LEASE (this "AMENDMENT") is made and entered
into as of September 8, 1997, by and between MISSION VACAVILLE LIMITED
PARTNERSHIP, a California limited partnership ("LANDLORD"), and BIOSOURCE
TECHNOLOGIES, INC., a California corporation ("Tenant"), with reference to the
following Recitals:

                                   BACKGROUND

            A. Landlord (as successor in interest to Sepulveda Properties, Inc.)
        and Tenant are currently parties to the Vaca Valley Business Park Office
        Lease dated as of October 15, 1987, as amended by the First Amendment
        thereto dated as of July 1, 1989, the Second Amendment thereto dated as
        of January 1, 1992, the Third Amendment thereto dated as of June 30,
        1992 (the "Third Amendment"), the Fourth Amendment thereto dated as of
        November 22, 1995 and the Fifth Amendment thereto dated as of November
        22, 1996 (as amended, the "Lease"), pursuant to which Tenant currently
        leases certain office space within the Interchange Business Park -
        Vacaville, at 3333 Vaca Valley Parkway, Vacaville, California 95688,
        which space is more particularly described in the, Lease (the
        "Premises"). (The Tenant was formerly named Biosource Genetics
        Corporation.)

            B. Tenant now desires to expand the Premises by leasing from
        Landlord approximately 9,168 additional square feet in the Building more
        particularly described on the floor plan attached hereto as Exhibit A
        (the "Expansion Space"). Landlord has agreed to lease the Expansion
        Space to Tenant on the terms and conditions of the Lease and this Sixth
        Amendment.

             NOW, THEREFORE, in consideration of the foregoing Recitals, the
agreements set forth herein, and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, Landlord and Tenant
hereby agree as follows:

        1. DEFINITIONS. All initially capitalized terms not otherwise defined
herein shall have the meanings set forth in the Lease. From and after the date
hereof, references to the Lease shall mean the Lease as amended hereby. In the
event of any inconsistency between the terms of the Lease as amended to date and
the terms of this Amendment, the terms of this Amendment shall prevail.

        2. EXPANSION OF PREMISES. Landlord hereby leases the Expansion Space to
Tenant on the terms set forth herein, and Tenant hereby agrees to lease the
Expansion Space from Landlord, effective as of the date of this Amendment (the
"Expansion Date"). From and after the Expansion Date, the Expansion Space shall
become a part of the Premises for all purposes under the Lease. Tenant
acknowledges that Tenant has inspected the Expansion Space, is familiar with the
condition thereof and hereby accepts the Expansion Space in an "AS IS" physical
condition and in an "AS IS" state of repair. Tenant further acknowledges that,
other than Landlord's obligation to fund the Improvement Allowance as provided
in Section 5, below, Landlord is not obligated to construct, install, or pay for
any improvements of any kind whatsoever to the


                                Sixth Amendment
<PAGE>   58

Expansion Space to prepare it for Tenant's occupancy.

        3. RENT. The Base Rent payable by Tenant pursuant to Section 4 of the
Lease is increased to $22,348.00 per month effective as of the Expansion Date.

        4. TENANT'S SHARE. The Tenant's Share under the Lease is increased to
56.17% effective as of the Expansion Date.

        5. Effective as of the Expansion Date, the following is hereby added to
the Lease at the end of Section 5(a):

           Notwithstanding anything to the contrary in this Lease,
           "Operating Expenses" shall not include the cost of janitorial
           and cleaning service expenses related to the portion of the
           common areas shaded on Exhibit B hereto.

        6. IMPROVEMENT ALLOWANCE.

            (a) Tenant shall receive a tenant improvement allowance with respect
to the Expansion Space in the amount of $165,000 (the "IMPROVEMENT ALLOWANCE"),
which shall be disbursed to Tenant by Landlord only for reimbursement of the
costs of construction of improvements performed by Tenant in the Expansion Space
within two years after the Expansion Date (including the cost of preparing space
plans and other plans, specifications and working drawings necessary to
construct such improvements). For the sake of clarity, Tenant hereby
acknowledges that (i) no portion of the Improvement Allowance may be used to
reimburse the cost of personal property or trade fixtures installed in the
Premises, (ii) no further disbursements of the Improvement Allowance shall be
made with respect to improvement work performed more than two years after the
Expansion Date and (iii) Tenant shall not be entitled to any credit or payment
for any unused portions of the Improvement Allowance.

            (b) The Improvement Allowance shall be paid to Tenant in
installments as tenant's work progresses, but in no event more frequently than
once per month. During construction of Tenant's improvement work, Tenant shall,
on or before the tenth day of each calendar month, deliver to Landlord for
approval a request for reimbursement which -shall be accompanied by (i) paid
invoices for the work performed since the last reimbursement, with sufficient
back-up billings from subcontractors, suppliers and providers of services to
give Landlord a complete financial accounting regarding each invoice to be
reimbursed by Landlord, (ii) if tenant's improvements involve plans, a
certificate signed by tenant's architect certifying that the work represented by
the aforesaid invoices has been satisfactorily completed in accordance with
tenant's plans, (iii) conditional lien waivers by Tenant's contractors,
subcontractors and materialmen for all such work and final lien waivers for all
completed work to the extent available, and (iv) with respect to the
disbursement of the final 10% of the Improvement Allowance, final lien waivers
and a general release from the contractor, subcontractors and materialmen
regarding tenant's improvement work along with final governmental approvals of
tenant's improvement work (to the extent such approvals are required by
applicable law). Landlord shall pay to Tenant (and not to the persons submitting
invoices), the amounts paid by Tenant for such invoices, up to the aggregate
amount of the Improvement


                                Sixth Amendment-2
<PAGE>   59

Allowance for the space in which tenant's improvement work is being performed,
within 30 days after receipt of all of the information and documentation
specified above with respect to such reimbursement request. Landlord shall have
the right to inspect the Premises to confirm the completion of the work covered
by such invoices. Customary and appropriate procedures shall be followed by
Tenant to assure satisfaction or waiver of any potential mechanics' lien claims.
Notwithstanding anything to the contrary in the foregoing, Landlord shall have
no obligation to disburse any portion of the Improvement Allowance for any
purposes upon the occurrence and during the continuance of a default by Tenant
under the terms of the Lease.

            (c) Landlord's agreement to fund the Improvement Allowance does not
constitute a waiver of the requirements of Section 10 (Alterations) of the
Lease; Tenant shall seek Landlord's prior written consent (which shall not be
unreasonably withheld) to any alterations or improvements to the Premises
(including the Expansion Space), and shall otherwise comply with all
requirements of Section 10 of the Lease. In connection with any alterations or
improvement work performed by Tenant in the Premises (including the Expansion
Space): (i) Landlord shall be permitted to post notices of nonresponsibility on
the Premises; (ii) Tenant shall comply with all applicable laws, regulations and
codes; (iii) Tenant shall indemnify, defend (with counsel reasonably approved by
Landlord) and hold harmless Landlord from any cost, expense, claim, lien, loss,
damage or liability; (iv) all space plans, plans and specification and
mechanical and electrical drawings shall be subject to Landlord's review and
approval; and (v) Tenant shall submit final as-built plans to Landlord upon
completion of tenant's improvement work.

        7. EARLY TERMINATION OPTION DELETED. Paragraph F of Article III of the
Third Amendment, regarding early termination of the Lease, is hereby deleted.

        8. RATIFICATION. The Lease, as amended by this Amendment, is hereby
ratified and confirmed in all respects and remains in full force and effect.
This Amendment shall be effective as of the date hereof upon execution and
delivery by both Landlord and Tenant. Except for those provisions of the Lease
which are expressly amended hereby, the Lease remains unmodified and unchanged.

                         [SIGNATURES ON FOLLOWING PAGE]


                               Sixth Amendment-3
<PAGE>   60

        IN WITNESS WHEREOF, Tenant and Landlord have executed this Amendment as
of the date first above written.

                                "LANDLORD"

                                MISSION VACAVILLE LIMITED PARTNERSHIP,
                                a California limited partnership

                                By:   Mission Airport Park Development Company,
                                      a California corporation
                                      Its General Partner

                                      By:   /s/ Sunny Lee
                                            ------------------------
                                      Name:
                                           -------------------------
                                      Its:
                                           -------------------------

                                "TENANT"

                                BIOSOURCE TECHNOLOGIES, INC.,
                                a California corporation

                                By: /s/ David R. McGee
                                    --------------------------------
                                Name:
                                     -------------------------------
                                Its:
                                    --------------------------------


                               Sixth Amendment-4
<PAGE>   61

                                    EXHIBIT A

                               THE EXPANSION SPACE

                                 [See Attached]


                                  Exhibit A-1
<PAGE>   62

                          (Blueprint of Expanded Space)


                                  Exhibit A-2
<PAGE>   63

                                    EXHIBIT B

        (Common Area expenses will not include janitorial and cleaning service
expenses related to the area shown shaded on the attached)


                                  Exhibit B-1
<PAGE>   64

                          (Schematic of Corporate Park)


                                  Exhibit B-2
<PAGE>   65

                           SEVENTH AMENDMENT TO LEASE

        THIS SEVENTH AMENDMENT TO LEASE (this "AMENDMENT") is made and entered
into as of July 27, 1998, by and between MISSION VACAVILLE LIMITED PARTNERSHIP,
a California limited partnership ("LANDLORD"), and BIOSOURCE TECHNOLOGIES, INC.,
a California corporation ("Tenant"), with reference to the following Recitals:

                                   BACKGROUND

            A. Landlord and Tenant are currently parties to the Vaca Valley
        Business Park Office Lease dated as of October 15, 1987, as amended by
        the First Amendment thereto dated as of July 1, 1989, the Second
        Amendment: thereto dated as of January 1, 1992, the Third Amendment
        thereto dated as of June 30, 1992, the Fourth Amendment thereto dated as
        of November 22, 1995, the Fifth Amendment thereto dated as of November
        22, 1996 and the Sixth Amendment thereto dated as of September 8, 1997
        (as amended, the "Lease"), pursuant to which Tenant currently leases
        certain office space within the Interchange Business Park - Vacaville
        ("BUILDING"), at 3333 Vaca Valley Parkway, Vacaville, California 95688,
        which space is more particularly described in the Lease (the
        "PREMISES").

            B. The Babcock & Wilcox Company ("BABCOCK") currently leases from
        Landlord certain space in the Building known as Suite 300, consisting of
        approximately 7,995 rentable square feet and hatch-marked on the floor
        plan attached hereto at Exhibit A (the "1998 EXPANSION SPACE"). Babcock
        wishes to terminate its lease, and Tenant wishes to expand into the 1998
        Expansion Space.

            C. Landlord has agreed to lease the 1998 Expansion Space to Tenant
        on the terms and conditions of the Lease and this Seventh Amendment.
        Landlord has agreed to terminate Babcock's lease, pursuant to a separate
        agreement, effective on the Expansion Date defined in Section 2, below.

            NOW, THEREFORE, in consideration of the foregoing Recitals, the
agreements set forth herein, and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, Landlord and Tenant
hereby agree as follows.

        1. DEFINITIONS. All initially capitalized terms not otherwise defined
herein shall have the meanings set forth in the Lease. From and after the date
hereof, references to the Lease shall mean the Lease as amended hereby. In the
event of any inconsistency between the terms of the Lease as amended to date and
the terms of this Amendment, the terms of this Amendment shall prevail.

        2. EXPANSION OF PREMISES. Landlord hereby leases the 1998 Expansion
Space to Tenant on the terms set forth herein, and Tenant hereby agrees to lease
the 1998 Expansion Space from Landlord, effective as of September, 1, 1998 (the
"EXPANSION DATE"). From and after the Expansion Date, the 1998 Expansion Space
shall become a part of the Premises for all purposes under the Lease. Tenant
acknowledges that Tenant has inspected the 1998 Expansion Space, is familiar
with the condition thereof and hereby accepts the 1998 Expansion Space in an


                               Seventh Amendment
<PAGE>   66

"AS IS" physical condition and in an "AS IS" state of repair. Tenant further
acknowledges that Landlord is not obligated to construct, install, or pay for
any improvements of any kind whatsoever to the 1998 Expansion Space to prepare
it for Tenant's occupancy.

        3. Rent.

            (a) The Base Rent payable by Tenant pursuant to Section 4 of the
Lease is increased to $29,779.55 per mouth effective as of the Expansion Date.

            (b) The portion of the Base Rent attributable to the 1998 Expansion
Space defined in this Amendment (which portion currently equals $7,431.55) (the
"1998 Expansion Space Rent") shall be subject to annual increases as follows:

                 On January 1, 1999 and on January lst of each calendar year of
            the Lease Term thereafter ("ADJUSTMENT DATE"), the Base Rent in
            effect immediately prior to such Adjustment Date shall increase by
            the percentage increase of the CPI published for January 1st of the
            preceding calendar year, provided, however, that (i) if such
            percentage increase is zero or less, the Base Rent shall neither
            increase nor decrease and (ii) if such percentage increase is
            greater than 8% the Base Rent shall increase by 8 %. The Base Rent
            as adjusted shall be the Base Rent due under the Lease until the
            next Adjustment Date and shall be the basis upon which the next CPI
            adjustment shall be made. As used herein, "CPI" means the Consumer
            Price Index for All Urban Consumers for San Francisco--Oakland, as
            published by the Bureau of Labor Statistics of the U.S. Department
            of Labor.

        4. TENANT'S SHARE. The Tenant's Share under the Lease is increased to
72.24% effective as of the Expansion Date. However, with respect to the portion
of Tenant's Share that is attributable to the 1998 Expansion Space (the portion
equaling 16.08 % which is included in the 72.24 %), the first sentence of
Section 5(a) of the lease is reinstated as originally drafted (and the
replacement sentence provided in the Third Amendment to the Lease shall not
apply).

        5. IMPROVEMENT ALLOWANCE FULLY FUNDED. Tenant hereby confirms that only
$8,544 of the "Improvement Allowance" defined in the Sixth Amendment to the
Lease remains undisbursed, and that the remaining sum may be used only for the
Expansion Space defined in the Sixth Amendment, in accordance with the paragraph
6 of Sixth Amendment, and that Tenant has no other right to receive any
improvement allowance of any kind from Landlord.

        6. RATIFICATION. The Lease, as amended by this Amendment, is hereby
ratified and confirmed in all respects and remains in full force and effect.
This Amendment shall be effective as of the date hereof upon execution and
delivery by both Landlord and Tenant. Except for those provisions of the Lease
which are expressly amended hereby, the Lease remains unmodified and unchanged.


                              Seventh Amendment-2
<PAGE>   67

        7. COUNTERPARTS. This Amendment may be executed in counterparts, each of
which shall be an original, but all of which together shall constitute one
agreement.

                         [SIGNATURES ON FOLLOWING PAGE]


                              Seventh Amendment-3
<PAGE>   68

        IN WITNESS WHEREOF, Tenant and Landlord have executed this Amendment as
of die date first above written.

                              "LANDLORD"

                              MISSION VACAVILLE LIMITED PARTNERSHIP,
                              a California limited partnership

                              By:     Mission Airport Park Development Company,
                                      a California corporation
                                      Its General Partner

                                      By:./s/ Charles W. Johnson
                                          ----------------------------
                                      Name:
                                           ---------------------------
                                      Its:
                                           ---------------------------

                              "TENANT"

                              BIOSOURCE TECHNOLOGIES, INC.,
                              a California corporation

                              By:/s/ David R. McGee
                                 ---------------------------------
                              Name:
                                   -------------------------------
                              Its:
                                   -------------------------------


                              Seventh Amendment-4
<PAGE>   69

                                    EXHIBIT A

                            THE 1998 EXPANSION SPACE

                                 [See Attached]


                                  Exhibit A-1
<PAGE>   70

                          (Blueprint of Expanded Area)


                                  Exhibit A-2
<PAGE>   71

                            EIGHTH AMENDMENT TO LEASE



        THIS EIGHTH AMENDMENT TO LEASE (this "Amendment") is made and entered
into as of May 19, 1999, by and between MISSION VACAVILLE LIMITED PARTNERSHIP, a
California limited partnership ("LANDLORD"), and BIOSOURCE TECHNOLOGIES, INC., a
California corporation ("Tenant"), with reference to the following Recitals:

                                   BACKGROUND

            A. Landlord and Tenant are currently parties to the Vaca Valley
Business Park Office Lease dated as of October 15, 1987, as amended by the First
Amendment thereto dated as of July 1, 1989, the Second Amendment thereto dated
as of January 1, 1992, the Third Amendment thereto dated as of June 30, 1992,
the Fourth Amendment thereto dated as of November 22, 1995, the Fifth Amendment
thereto dated as of November 22, 1996, the Sixth Amendment thereto dated as of
September 8, 1997 and the Seventh Amendment thereto dated as of July 25, 1998
(as amended, the "LEASE"), pursuant to which Tenant currently leases certain
office space within the Interchange Business Park - Vacaville ("BUILDING"), at
3333 Vaca Valley Parkway, Vacaville, California 95688, which space is more
particularly described in the Lease (the "PREMISES").

            B. Tenant desires to perform certain improvements described on
Exhibit A attached hereto, involving the installation of a backup generator and
related equipment (the "Generator Improvements") to be located on a concrete pad
to be built on the exterior of the Building as shown on Exhibit B attached
hereto (the "GENERATOR SPACE").

            C. Landlord has agreed to permit construction of the Generator
improvements on the Generator Space, subject to the terms of this Amendment.

            NOW, THEREFORE, in consideration of the foregoing Recitals, The
agreements set forth herein, and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, Landlord and Tenant
hereby agree as follows:

        1. DEFINITIONS. All initially capitalized terms not otherwise defined
herein shall have the meanings set forth in the Lease. From and after the date
hereof, references to the Lease shall mean the Lease as amended hereby. In the
event of any inconsistency between the terms of the Lease as amended to date and
the terms of this Amendment, the terms of this Amendment shall prevail.

        2. CONSENT TO THE IMPROVEMENTS. Landlord hereby consents to Tenant's
installation of the Generator Improvements, provided Tenant shall perform and
the complete the Generator Improvements at its sole cost, in a good and
workmanlike manner, and in compliance with all applicable laws, codes and the
recommendations of all governmental agencies. Tenant shall be solely responsible
for obtaining any necessary permits for installation of the Generator
Improvements. Tenant shall be solely responsible for performing any improvements
or system upgrades required by any governmental agency in the course of granting
permits or performing


                                Eighth Amendment
<PAGE>   72

inspections for the Generator Improvements.

        3. USE OF GENERATOR SPACE. The generator shall be maintained for backup
purposes only, and may be operated only when electricity utility service to the
Premises fails or for maintenance of the generator (including brief start-ups
for maintenance check purposes). Lessee shall have a non-exclusive license to
use the Generator Space for the installation, use and maintenance of the
Generator Improvements. Landlord is under no obligation to prevent Landlord's
tenants or others from gaining access to the Generator Space, and has no
obligation to keep the Generator Improvements safe from harm by Landlord's
tenants or others.

        4. OWNERSHIP, MAINTENANCE AND REPAIR OF THE IMPROVEMENTS. The generator
itself will remain the property of Tenant after installation thereof, to be
removed by Tenant at the expiration of the Term, at which time Tenant shall
repair any and all damage caused by such removal. Tenant will purchase the
generator free of liens; the generator will not be leased and tenant will not
allow a financing statement or fixture filing to be filed or recorded with
respect to the generator. With the exception of the generator itself, all of the
Generator Improvements will be the property of Landlord, subject to the terms of
Paragraph 10 of the Lease. Tenant is solely responsible for maintaining the
Generator Improvements at Tenant's expense. If the Generator Improvements are
damaged or destroyed by any cause then, notwithstanding anything to the contrary
in paragraph 11 of the Lease, Landlord will have no responsibility to repair or
restore the Generator Improvements, and Tenant shall completely restore the
Generator Improvements at its sole cost and expense.

        5. INSURANCE. Landlord will have no responsibility to insure the
Generator Improvements. Tenant's insurance obligations under Paragraph 12 of the
Lease shall extend to and include the Generator Improvements. Tenant shall
reimburse Landlord, within ten (10) days after demand and presentation of an
invoice therefor, for any increase in premiums for insurance carried by Landlord
in connection with the Building, either now or in the future, resulting from the
generator or related equipment.

        6. COMPLIANCE WITH LAWS.

            6.1 Without limiting any other obligation of Tenant under the Lease
to comply with laws, Tenant shall, at Tenant's sole cost and expense, fully,
diligently and in a timely manner, comply with all "APPLICABLE REQUIREMENTS,"
which term is used in this Lease to mean all laws, rules, regulations,
ordinances, directives, covenants, easements and restrictions of record,
permits, the requirements of any applicable fire insurance underwriter or rating
bureau, and the recommendations of Landlord's engineers and/or consultants,
relating in any manner to the Premises or the Generator Space (including but not
limited to matters pertaining to (i) industrial hygiene, (ii) environmental
conditions on, in, under or about the Premises or the Generator Space, including
soil and groundwater conditions, and (iii) the use, generation, manufacture,
production, installation, maintenance, removal, transportation, storage, spill,
or release of any Hazardous Substance, as defined in subparagraph 6.3, below),
now in effect or which may hereafter come into effect. Tenant shall, within five
(5) days after receipt of Landlord's written request, provide Landlord with
copies of all documents and information, including but not limited to permits,
registrations, manifests, applications, reports and certificates, evidencing
Tenant's compliance with any Applicable Requirements specified by


                               Eighth Amendment-2
<PAGE>   73

Landlord, and shall immediately upon receipt, notify Landlord in writing (with
copies of any documents involved) of any threatened or actual claim, notice,
citation, warning, complaint or report pertaining to or involving failure by
Tenant or the Premises or the Generator Space to comply with any Applicable
Requirements.

            6.2 Landlord, Landlord's agents, employees, contractors and
designated representatives, and the holders of any mortgages, deeds of trust or
ground leases on the Premises ("LENDERS") shall have the right to enter the
Premises and the Generator Space at any Time in the case of an emergency, and
otherwise at reasonable times, for the purpose of inspecting the condition of
the Premises and the Generator Space and for verifying compliance by Tenant with
this Amendment and all Applicable Requirements, and Landlord shall be entitled
to employ experts and/or consultants in connection therewith to advise Landlord
with respect to Tenant's activities, including but nor limited to Tenant's
installation, operation, use, monitoring, maintenance, or removal of any
Hazardous Substance on or from the Premises or the Generator Space. The costs
and expenses of any such inspections shall be paid by the parry requesting same,
unless a Default or Breach of this Amendment by Tenant or a violation of
Applicable Requirements or a contamination, caused or materially contributed to
by Tenant, is found to exist or to be imminent, or unless the inspection is
requested or ordered by a governmental authority as the result of any such
existing or imminent violation or contamination. In such case, Tenant shall upon
request reimburse Landlord or Landlord's Lender, as the case may be, for the
costs and expenses of such inspections.

            6.3 The term "HAZARDOUS SUBSTANCE" as used in this Amendment shall
mean any product, substance, chemical, material or waste whose presence, nature,
quantity and/or intensity of existence, use, manufacture, disposal,
transportation, spill, release or effect, either by itself or in combination
with other materials expected to be on the Premises or the Generator Space, is
either, (i) potentially injurious to the public health, safety or welfare, the
environment, or the premises; (ii) regulated or monitored by any governmental
authority; or (iii) a basis for potential liability of Lessor to any
governmental agency or third parry under any applicable Statute or common law
theory. Hazardous Substance shall include, but not be limited to, hydrocarbons,
petroleum, gasoline, crude oil or any products or by-products thereof Lessee
shall not engage m any activity in or about the Premises or the Generator Space
which constitutes a Reportable Use (as hereinafter defined) of Hazardous
Substances without the express prior written consent of Lessor and compliance in
a timely manner (at Lessee's sole cost and expense) with all Applicable
Requirements. "REPORTABLE Use" shall mean (i) the installation or use of any
above or below ground storage tank, (ii) the generation, possession, storage,
use, transportation, or disposal of a Hazardous Substance that requires a permit
from, or with respect to which a report, notice, registration or business plan
is required to be filed with, any governmental authority, and (iii) the presence
in, on or about the Premises or the Generator Space of a Hazardous Substance
with respect to which any Applicable Laws require that a notice be given to
persons entering or occupying the Premises or the Generator Space or neighboring
properties. Notwithstanding the foregoing, Lessee may, without Lessor's prior
consent, but upon notice to Lessor and in compliance with all Applicable
Requirements, use any ordinary and customary materials reasonably required to be
used by Lessee in the normal course of the Permitted Use, so long as such use is
nor a Reportable Use and does not expose the Premises, the Generator Space or
neighboring properties to any meaningful risk of contamination or damage or
expose Lessor to any liability therefor. In addition, Lessor may (but without
any obligation to do


                               Eithth Amendment-3
<PAGE>   74

 .so) condition its consent to any Reportable Use of any Hazardous Substance by
Lessee upon Lessee's giving Lessor such additional assurances as Lessor, in its
reasonable discretion, deems necessary to protect itself, The public, the
Premises and the environment against damage, contamination or injury and/or
liability therefor.

            6.4 DUTY TO INFORM LESSOR. If Lessee knows, or has reasonable cause
to believe, that a Hazardous Substance has come to be located in, on, under or
about the Premises, the Generator Space or the Building, other than as
previously consented to by Lessor, Lessee shall immediately give Lessor written
notice thereof, together with a copy of any statement, report, notice,
registration, application, permit, business plan, license, claim, action, or
proceeding given to, or received from any governmental authority or private
parry concerning the presence, spill, release, discharge of, or exposure to,
such Hazardous Substance including bur not limited to all such documents as may
be involved in any Reportable Use involving the Premises or the Generator Space.
Lessee shall not cause or permit any Hazardous Substance to be spilled or
released in, on, under or about the Premises or the Generator Space (including,
without limitation, through the plumbing or sanitary sewer system)-

            6.5 Tenant shall indemnify, protect, defend and hold Landlord, its
agents, employees, lenders and ground Landlord, if any, and the Premises,
harmless from and against any and all damages, liabilities, judgments, costs,
claims, liens, expenses, penalties, loss of permits and attorneys' and
consultants' fees arising out of or involving the Generator Improvements.
Tenant's obligations under this Paragraph 6 shall include, but not be limited
to, the effects of any contamination or injury to person, property or the
environment created or suffered by Tenant, and the cost of investigation
(including consultants' and attorneys' fees and testing), removal, remediation,
restoration and/or abatement thereof or of any contamination therein involved,
and shall survive the expiration or earlier termination of this Lease.

        7. RATIFICATION. The Lease, as amended by this Amendment, is hereby
ratified and confirmed in all respects and remains in full force and effect.
This Amendment shall be effective as of the date hereof upon execution and
delivery by both Landlord and Tenant. Except for those provisions of the Lease
which are expressly amended hereby, the Lease remains unmodified and unchanged.

        8. COUNTERPARTS. This Amendment may be executed in counterparts, each of
which shall be an original, but all of which together shall constitute one
agreement.

                         [SIGNATURES ON FOLLOWING PAGE]


                               Eighth Amendment-4
<PAGE>   75

        IN WITNESS WHEREOF, Tenant and Landlord have executed this Amendment as
of the date first above written.

                                "LANDLORD"

                                MISSION VACAVILLE LIMITED PARTNERSHIP,
                                a California limited partnership

                                By:  Mission Airport Park Development Company,
                                     a California corporation
                                     Its General Partner

                                     By:
                                        --------------------------------
                                     Name:
                                          ------------------------------
                                     Its:
                                          ------------------------------

                                "TENANT"

                                BIOSOURCE TECHNOLOGIES, INC.,
                                a California corporation

                                By:  /s/ David R. McGee
                                     -------------------------------
                                Name:
                                     -------------------------------
                                Its:
                                     -------------------------------


                               Eighth Amendment-5
<PAGE>   76

                                    EXHIBIT A

                      DESCRIPTION OF GENERATOR IMPROVEMENTS

                                 [SEE ATTACHED]


                                  Exhibit A-1
<PAGE>   77

                      (Photograph & Blueprint of Generator)


                                  Exhibit A-2
<PAGE>   78

                                    EXHIBIT B

                         DESCRIPTION OF GENERATOR SPACE

                                 [SEE ATTACHED]


                                  Exhibit B-1
<PAGE>   79

                      (Technical Drawing of the Generator)


                                  Exhibit B-2
<PAGE>   80

April 20, 1999

Charles W. Johnson
MISSION LAND COMPANY
61 Elliot Court
Simi Valley, CA  93065

        Re:     Tenant Improvement

Dear Mr. Johnson:

Pursuant to the terms of our lease, this letter is to notify you of Biosource's
proposed tenant improvements to the building at 333 Vaca Valley Parkway,
Vacaville, CA. Enclosed please find drawings outlining the installation of a
backup generator for all suites occupied by Biosource Technologies. This
generator would be located on the east side of the building near the electrical
room on a concrete pad surrounded by bollards and tied into the electrical
system via a new distribution board and automatic transfer switch.

We have taken the preliminary step of obtaining permits from the City of
Vacaville to insure their complete endorsement of the project. Impact to the
exterior of the building is limited because it will be partially hidden by an
existing cinder block wall and we will match the paint to that of the building.
The generator itself will be removable if Biosource ever decides to relocate. At
that time, Mission Land would have the option of replacing the generator for use
by their new tenants or Biosource would return the parking lot to its original
condition by removing the pad and bollards and repaving.

We believe installation of the electrical connections for this generator will be
a valuable leasehold improvement to the property for Mission Land and once
completed, an immeasurable insurance policy against the possibility of Y2K
related power problems for Biosource. All construction is to code and all
permits required by the City of Vacaville have been obtained. The general
contractor will be Mark III Engineering Contractors of Sacramento.

We respectfully request your signature below, authorizing the improvements.
Please return the letter via FAX to (707) 446-397 and follow with the original
by mail. Thank you for your time and consideration, and please feel free to call
me if you have any questions.

Very Truly yours,

BIOSOURCE TECHNOLOGIES, INC.         The above-referenced improvements and
                                     additions are hereby approved

                                     MISSION LAND COMPANY
Knat Holben
Operations Manager

KH/slh                               By
Enclosures                             -------------------------------------
                                                                 (Signature)

                                       -------------------------------------
                                              (Printed Name & Title)

                                       -------------------------------------
                                                                      (Date)