STOCK PURCHASE AGREEMENT

     Agreement  dated as of  September  27, 1996  between  Monsanto  Company,  a
Delaware  corporation  ("Monsanto"),  and Calgene,  Inc., a Delaware corporation
("Calgene").

     In  consideration  of the mutual  promises and covenants  contained in this
Agreement, the parties hereto agree as follows:

     1. Sale of Shares.  Subject to the terms and conditions of this  Agreement,
at the Closing (as defined below),  Calgene will sell and issue to Monsanto, and
Monsanto will purchase,  6,250,000 shares (the "Shares") of Common Stock,  $.001
par value per share, of Calgene ("Common Stock") for an aggregate purchase price
of  $50,000,000.  The number of Shares shall be  appropriately  adjusted for any
stock splits, stock dividends or similar events affecting the Common Stock after
the date hereof and prior to the Closing.

     2. The Closing.

     (a) The closing  ("Closing")  of the sale and  purchase of the Shares under
this  Agreement  shall  take  place at the  offices  of Hale and Dorr,  60 State
Street, Boston, Massachusetts at 9:00 a.m. on the first date on which all of the
conditions set forth in Sections 6 and 7 have been satisfied or duly waived.  At
the Closing,  Calgene shall  deliver to Monsanto a  certificate  for the Shares,
registered in the name of Monsanto,  against  payment to Calgene of the purchase
price therefor, by wire transfer.

     (b)  Either  Monsanto  or  Calgene  may  terminate  this  Agreement  if the
conditions  set forth in Sections 6 and 7 have not been satisfied or duly waived
prior to January 31, 1997, other than by reason of a breach of this Agreement by
the terminating party.

     3.  Representations  of  Calgene.  Subject  to and except as  disclosed  by
Calgene in the disclosure  schedule  provided by Calgene to Monsanto on the date
hereof (the "Disclosure  Schedule"),  Calgene hereby  represents and warrants as
follows:

     3.1  Organization  and Standing.  Calgene is a corporation  duly organized,
validly  existing and in good  standing  under the laws of the State of Delaware
and has full corporate  power and authority to conduct its business as presently
conducted  and as proposed to be  conducted  by it and to enter into and perform
this Agreement and to carry out the transactions contemplated by this Agreement.
Calgene is duly qualified to do business as a foreign corporation and is in good
standing in the State of California and in every other jurisdiction in which the
failure  to so  qualify  would have a  material  adverse  effect on the  assets,
business,  results of operations or financial condition of Calgene.  Calgene has
furnished  to  Monsanto  true  and  complete   copies  of  its   Certificate  of
Incorporation and By-Laws, each as amended to date and presently in effect.
<PAGE>

     3.2  Capitalization.  The authorized  capital stock of Calgene  consists of
80,000,000  shares of Common Stock, of which  60,443,115  shares were issued and
outstanding  as of August 31, 1996, and  10,000,000  shares of Preferred  Stock,
$.001 par value per share,  none of which shares are issued or outstanding.  All
of the issued and  outstanding  shares of Common Stock have been duly authorized
and validly issued and are fully paid and nonassessable.  Except as set forth in
Section 3.2 of the Disclosure Schedule hereto or provided in this Agreement, (i)
no  subscription,   warrant,   option,   convertible  security  or  other  right
(contingent  or otherwise) to purchase or acquire any shares of capital stock of
Calgene is authorized or outstanding, (ii) Calgene has no obligation (contingent
or otherwise) to issue any subscription,  warrant, option,  convertible security
or other  such right or to issue or  distribute  to holders of any shares of its
capital  stock any  evidences of  indebtedness  or assets of Calgene,  and (iii)
Calgene has no  obligation  (contingent  or  otherwise)  to purchase,  redeem or
otherwise  acquire any shares of its capital stock or any interest therein or to
pay any dividend or make any other distribution in respect thereof.

     3.3 Issuance of Shares. The Shares, when issued, sold and delivered against
payment  therefor in accordance with the provisions of this  Agreement,  will be
duly and validly issued, fully paid and non-assessable.

     3.4 Authority for  Agreement.  The execution,  delivery and  performance by
Calgene of this  Agreement and the Amended and Restated  Stockholders  Agreement
attached hereto as Exhibit A (the "Amendment"),  and the consummation by Calgene
of the transactions  contemplated hereby and thereby,  have been duly authorized
by all necessary corporate action, subject to (i) approval of the Certificate of
Amendment  attached hereto as Exhibit B (the  "Certificate of Amendment") by the
stockholders  of  Calgene  holding a  majority  of the  shares  of Common  Stock
outstanding,  and (ii)  approval of the issuance of the Shares to Monsanto,  the
Amendment  and this  Agreement  by the holders of a majority of the  outstanding
shares of Common Stock present or represented at the meeting of  stockholders to
be  held  for  such  purpose  (the  "Stockholders  Meeting"),  excluding  broker
non-votes  and shares  held by  Monsanto;  (such  approvals  being  referred  to
collectively as the "Requisite Stockholder Approval").  This Agreement has been,
and the  Amendment  as of the Closing will be, duly  executed  and  delivered by
Calgene. This Agreement constitutes, and the Amendment will constitute as of the
Closing,  valid and binding  obligations  of Calgene,  enforceable in accordance
with  their   respective   terms.  The  execution  of  and  performance  of  the
transactions  contemplated  by this  Agreement and the Amendment and  compliance
with their  provisions by Calgene will not violate any provision of law and will
not  conflict  with or result in any breach of any of the terms,  conditions  or
provisions  of, or  constitute a default  under,  or require a consent or waiver
under,  its Certificate of  Incorporation or ByLaws (each as amended to date) or

<PAGE>

any indenture,  lease, agreement or other instrument to which Calgene is a party
or by which it or any of its  properties  is  bound,  or any  decree,  judgment,
order, statute, rule or regulation applicable to Calgene.

     3.5 Governmental Consents. No consent, approval, order or authorization of,
or  registration,  qualification,  designation,  declaration or filing with, any
governmental authority is required on the part of Calgene in connection with the
execution and delivery of this Agreement, the issuance, sale and delivery of the
Shares,  or  the  other  transactions  to be  consummated  at  the  Closing,  as
contemplated  by this  Agreement,  except (i) the filing with the  Secretary  of
State of Delaware of the  Certificate  of  Amendment  and (ii)  compliance  with
Hart-Scott-Rodino  Antitrust  Improvements  Act of 1976,  as  amended  (the "HSR
Act"), and (iii)  compliance with the applicable  requirements of the Securities
Exchange Act of 1934 (the "Exchange Act").

     3.6 Reports and Financial  Statements.  Calgene has previously furnished to
Monsanto  complete and accurate copies,  as amended or supplemented,  of its (a)
Annual  Report on Form 10-K for the fiscal  year ended June 30,  1995,  as filed
with the Securities and Exchange  Commission (the "SEC"),  (b) proxy  statements
relating to all meetings of its  stockholders  (whether annual or special) since
June 30, 1995 and (c) all other reports or registration  statements,  other than
Registration  Statements  on Form S-8,  filed by Calgene with the SEC since June
30, 1995 (such annual reports,  proxy  statements,  registration  statements and
other  filings,  together  with  any  amendments  or  supplements  thereto,  are
collectively  referred to herein as the "Calgene Reports").  The Calgene Reports
constitute  all of the  documents  filed or required to be filed by Calgene with
the SEC since June 30, 1995, other than any Registration  Statement on Form S-8.
As of their  respective  dates,  the Calgene  Reports did not contain any untrue
statement  of a material  fact or omit to state a material  fact  required to be
stated  therein or necessary  to make the  statements  therein,  in light of the
circumstances under which they were made, not misleading.  The audited financial
statements and unaudited interim financial statements of Calgene included in the
Calgene Reports (together,  the "Financial Statements") (i) comply as to form in
all material respects with applicable accounting  requirements and the published
rules and regulations of the SEC with respect  thereto,  (ii) have been prepared
in  accordance  with United  States  generally  accepted  accounting  principles
("GAAP")  applied on a consistent  basis  throughout the periods covered thereby
(except as may be indicated therein or in the notes thereto,  and in the case of
quarterly  financial  statements,  as  permitted by Form 10-Q under the Exchange
Act),  and (iii)  fairly  present  in all  material  respects  the  consolidated
financial  condition,  results of operations and cash flows of Calgene as of the
respective dates thereof and for the periods referred to therein.
<PAGE>

     3.7  Litigation.  Section 3.7 of the Disclosure  Schedule  identifies,  and
contains a brief description of, (a) any unsatisfied  judgment,  order,  decree,
stipulation  or  injunction  and  (b)  any  claim,   complaint,   action,  suit,
proceeding,  hearing or investigation of or in any court or governmental  agency
or before any  arbitrator to which  Calgene or any  subsidiary is a party or, to
the knowledge of Calgene,  is threatened to be made a party,  in each case which
could  have a  material  adverse  effect  on  the  assets,  business,  financial
condition or results of operations of Calgene.

     4.  Representations  of  Monsanto.  Monsanto  represents  and  warrants  as
follows:

     4.1  Investment.  Monsanto is acquiring  the Shares for its own account for
investment  and  not  with a view  to,  or for  sale  in  connection  with,  any
distribution  thereof, nor with any present intention of distributing or selling
the same.

     4.2  Authority.  Monsanto has full power and authority to enter into and to
perform this  Agreement and the Amendment in  accordance  with their  respective
terms.

     4.3  Experience.  Monsanto  has  sufficient  knowledge  and  experience  in
investing in companies similar to Calgene so as to be able to evaluate the risks
and merits of its  investment  in Calgene  and is able  financially  to bear the
risks thereof.

     5. Covenants.

     5.1 Best Efforts.

     (a) Each of the parties shall use its  respective  best efforts to take all
actions and to do all things  necessary,  proper or advisable to consummate  the
transactions  contemplated  by this Agreement.  Without  limiting the foregoing,
Monsanto  shall  vote  all of  its  shares  of  Common  Stock  in  favor  of the
Certificate of Amendment at the Stockholders Meeting.

     (b) At the  Closing,  Monsanto  and Calgene  shall  execute and deliver the
Amendment.

     (c) At the  Closing,  Monsanto  and  Calgene  shall  execute a  certificate
setting forth the "Effective Date Percentage" as defined in the Amendment.

     5.2 Stockholders Meeting, Proxy Statement.

     (a) As soon as  practicable,  Calgene  shall  prepare and file with the SEC
under the Exchange Act preliminary  proxy materials (the "Proxy  Statement") for
the purpose of soliciting  proxies from its stockholders to vote in favor of (i)
the issuance of the Shares,  (ii) the  Certificate  of Amendment,  and (iii) the

<PAGE>

election of the directors contemplated by the Amendment (the "Proposals") at the
Stockholders Meeting.  Calgene shall promptly respond to any SEC comments on the
Proxy  Statement and shall otherwise use its best efforts to resolve as promptly
as practicable all SEC comments to the  satisfaction  of the SEC.  Calgene shall
furnish to Monsanto and its counsel copies of the  preliminary  Proxy  Statement
prior to filing it with the SEC and copies of the final Proxy Statement prior to
mailing it to stockholders.

     (b) Promptly following the resolution to the satisfaction of the SEC of all
SEC comments on the Proxy  Statement (or the  expiration  of the ten-day  period
under Rule  14a-6(a)  under the Exchange Act, if no SEC comments are received by
such date), Calgene shall distribute the Proxy Statement to its stockholders and
solicit proxies from its stockholders to vote in favor of the Proposals.

     (c) Calgene shall comply with all applicable  provisions of and rules under
the  Exchange  Act  and  all  applicable  provisions  of  the  Delaware  General
Corporation  Law  in the  preparation,  filing  and  distribution  of the  Proxy
Statement,  the solicitation of proxies thereunder,  and the calling and holding
of the  Stockholders  Meeting.  Without  limiting the  foregoing,  Calgene shall
ensure  that  the  Proxy  Statement  does  not,  as of the  date on  which it is
distributed to its stockholders, and as of the date of the Stockholders Meeting,
contain any untrue statement of a material fact or omit to state a material fact
necessary in order to make the  statements  made, in light of the  circumstances
under which they were made, not  misleading  (provided that Calgene shall not be
responsible for the accuracy or  completeness  of any  information  furnished by
Monsanto in writing for inclusion in the Proxy Statement).

     5.3 HSR Act. Each of the parties shall promptly file any  Notification  and
Report  Forms and  related  material  that it may be  required  to file with the
Federal  Trade  Commission  and the  Antitrust  Division  of the  United  States
Department of Justice under the HSR Act, shall use its best efforts to obtain an
early termination of the applicable  waiting period,  and shall make any further
filings or  information  submissions  pursuant  thereto  that may be  necessary,
proper or advisable.

     5.4 Pending Closing. Pending Closing Calgene shall carry on the business of
Calgene and its subsidiaries in the ordinary and regular course of business.

     6. Conditions to the Obligations of Monsanto. The obligation of Monsanto to
purchase Shares at the Closing is subject to the  fulfillment,  or the waiver by
Monsanto, of each of the following conditions on or before the Closing:

<PAGE>

     6.1 Accuracy of  Representations  and Warranties.  Each  representation and
warranty  contained  in  Section  3 shall be true and  correct  in all  material
respects  on and as of the  Closing  Date with the same  effect  as though  such
representation  and  warranty  had been made on and as of that date  (except for
representations  and warranties made as of a specific date,  which shall be true
and correct as of such date).

     6.2  Performance.  Calgene  shall  have  performed  and  complied  with all
agreements and conditions  contained in this Agreement  required to be performed
or complied with by Calgene prior to or at the Closing.

     6.3  Stockholder  Agreement.  The  Amendment  shall have been  executed and
delivered by Calgene.

     6.4 Certificate of Amendment.  The Certificate of Amendment shall have been
filed with the Secretary of State of Delaware.

     6.5 HSR Act. All applicable  waiting  periods (and any extensions  thereof)
under the HSR Act shall have expired or otherwise been terminated.

     6.6 No Injunctions or Other  Proceedings.  No temporary  restraining order,
preliminary or permanent  injunction or other order issued by any court or other
legal or regulatory restraint or prohibition  preventing the consummation of the
transactions  contemplated hereby shall have been issued and remain outstanding.
No investigation,  action,  suit or proceeding by any governmental or regulatory
commission,  agency, body or authority, and no action, suit or proceeding by any
other person or entity shall be pending on the Closing Date which challenges, or
might result in a challenge to, this Agreement or any transactions  contemplated
hereby,  or which  claims,  or might  give  rise to a claim  for,  damages  in a
material amount as a result of the consummation of this Agreement.

     6.7 NASDAQ. The Shares shall have been authorized for listing on the Nasdaq
National Market, subject to official notice of issuance.

     6.8 Opinion of Counsel.  Monsanto  shall have received an opinion from Hale
and Dorr, counsel for Calgene, dated the Closing Date, substantially in the form
attached hereto as Exhibit C.

     6.9 Stockholder Approval. The issuance of the Shares and the Certificate of
Amendment  shall  have  received  the  Requisite  Stockholder  Approval  and the
directors contemplated by the Amendment shall have been elected.

     6.10 Material  Adverse  Change.  Between the date of this Agreement and the
Closing Date there shall have been no materially adverse change in the position,
financial or otherwise,  or the  operations,  assets,  liabilities or results of

<PAGE>

operations of Calgene or its subsidiaries, provided that such a material adverse
change  shall not be deemed to have  occurred  as a result of any  change in the
customers,  pricing or sales volume of Calgene or any outcome of the  litigation
between Calgene and Enzo Biochem, Inc.

     6.11 No Change in  Capitalization.  Prior to Closing Calgene shall not have
issued,  agreed to issue or approve  the  issuance  of any shares of its capital
stock or any options,  warrants or other rights  entitling the holder thereof to
convert into or receive shares of Calgene capital stock, except for the grant of
options for Calgene  Common stock to employees and  consultants  in the ordinary
course of business and the issuance of shares of Calgene  Common Stock  pursuant
to the exercise of outstanding  options or warrants,  unless approved by Calgene
directors designated by Monsanto in writing.

     7.  Conditions to the  Obligations of Calgene.  The  obligations of Calgene
under this Agreement are subject to  fulfillment,  or the waiver by Calgene,  of
the following conditions on or before the Closing:

     7.1 Accuracy of Representations  and Warranties.  The  representations  and
warranties  of Monsanto  contained in Section 4 shall be true and correct on and
as of the Closing Date with the same effect as though such  representations  and
warranties had been made on and as of that date.

     7.2  Performance.  Monsanto  shall have  performed  and  complied  with all
agreements and conditions  contained in this Agreement  required to be performed
or complied with by Monsanto prior to or at the Closing.

     7.3  Stockholder  Agreement.  The  Amendment  shall have been  executed and
delivered by Monsanto.

     7.4 HSR Act. All applicable  waiting  periods (and any extensions  thereof)
under the HSR Act shall have expired or otherwise been terminated.

     7.5 No Injunction or Other  Proceedings.  No temporary  restraining  order,
preliminary or permanent  injunction or other order issued by any court or other
legal or regulatory restraint or prohibition  preventing the consummation of the
transaction  contemplated  hereby shall have been issued and remain outstanding.
No investigation,  action,  suit or proceeding by any governmental or regulatory
commission,  agency, body or authority, and no action, suit or proceeding by any
other person or entity shall be pending on the Closing Date which challenges, or
might result in a challenge to, this Agreement or any transactions  contemplated
hereby,  or which  claims,  or might  give  rise to a claim  for,  damages  in a
material amount as a result of the consummation of this Agreement.

<PAGE>

     7.6 Stockholder Approval. The issuance of the Shares and the Certificate of
Amendment shall have received the Requisite Stockholder Approval.

     8. Miscellaneous.

     8.1  Press  Releases  and  Announcements.  No party  shall  issue any press
release or public  disclosure  relating to the subject  matter of this Agreement
without the prior written approval of the other party;  provided,  however, that
any party may make any public  disclosure  it believes in good faith is required
by law or regulation (in which case the disclosing  party shall advise the other
party and provide it with a copy of the proposed  disclosure prior to making the
disclosure).

     8.2 No Third  Party  Beneficiaries.  This  Agreement  shall not  confer any
rights or remedies upon any person other than the parties.

     8.3 Entire Agreement.  This Agreement  (including the documents referred to
herein)  constitutes  the entire  agreement among the parties and supersedes any
prior  understandings,  agreements,  or representations by or among the parties,
written or oral, with respect to the subject matter hereof.

     8.4 Succession  and  Assignment.  This Agreement  shall be binding upon and
inure  to  the  benefit  of  the  parties  named  herein  and  their  respective
successors.  No party may assign  either  this  Agreement  or any of its rights,
interests,  or obligations  hereunder  without the prior written approval of the
other party.

     8.5   Counterparts.   This  Agreement  may  be  executed  in  two  or  more
counterparts,  each of  which  shall  be  deemed  an  original  but all of which
together shall constitute one and the same instrument.

     8.6 Headings. The section headings contained in this Agreement are inserted
for  convenience   only  and  shall  not  affect  in  any  way  the  meaning  or
interpretation of this Agreement.

     8.7  Governing  Law. This  Agreement  shall be governed by and construed in
accordance with the internal laws (and not the law of conflicts) of the State of
Delaware.

     8.8 Amendments and Waivers. The parties may mutually amend any provision of
this Agreement at any time prior to the Closing Date,  provided,  however,  that
any amendment effected subsequent to the Requisite Stockholder Approval shall be
subject to the restrictions  contained in the Delaware General  Corporation Law.
No waiver by any party of any default, misrepresentation,  or breach of warranty
or covenant hereunder,  whether intentional or not, shall be deemed to extend to
any prior or  subsequent  default,  misrepresentation,  or breach of warranty or
covenant  hereunder  or affect in any way any  rights  arising  by virtue of any
prior or subsequent such occurrence.
<PAGE>

     8.9  Severability.  Any term or provision of this Agreement that is invalid
or  unenforceable  in any  situation  in any  jurisdiction  shall not affect the
validity or  enforceability  of the remaining terms and provisions hereof or the
validity or  enforceability  of the  offending  term or  provision  in any other
situation  or in any other  jurisdiction.  If the final  judgment  of a court of
competent  jurisdiction declares that any term or provision hereof is invalid or
unenforceable,  the parties  agree that the court  making the  determination  of
invalidity  or  unenforceability  shall  have the  power to  reduce  the  scope,
duration, or area of the term or provision, to delete specific words or phrases,
or to replace  any invalid or  unenforceable  term or  provision  with a term or
provision that is valid and enforceable and that comes closest to expressing the
intention of the invalid or unenforceable term or provision,  and this Agreement
shall be  enforceable  as so modified  after the  expiration  of the time within
which the judgment may be appealed.

     8.10 Specific Performance. Each of the parties acknowledges and agrees that
the other party would be damaged  irreparably in the event any of the provisions
of this  Agreement are not performed in accordance  with their specific terms or
otherwise are breached.  Accordingly,  each of the parties agrees that the other
party shall be entitled to an injunction or injunctions  to prevent  breaches of
the provisions of this Agreement and to enforce  specifically this Agreement and
the terms and  provisions  hereof in any action  instituted  in any court of the
United States or any state thereof having  jurisdiction over the parties and the
matter,  in addition to any other remedy to which it may be entitled,  at law or
in equity.

     8.11  Construction.  The language used in this Agreement shall be deemed to
be the language chosen by the parties hereto to express their mutual intent, and
no rule of strict construction shall be applied against any party.

     IN WITNESS  WHEREOF,  the parties hereto have executed this Agreement as of
the date first above written.


                                         CALGENE, INC.

                                         By:/s/ Lloyd M. Kunimoto

                                         Title: President





<PAGE>









                                          MONSANTO COMPANY


                                          By:/s/ Hendrik A. Verfaillie

                                          Title: Executive Vice President