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                                                                  EXHIBIT 10.122





                              INVESTMENT AGREEMENT
                                       BY
                                  AND BETWEEN
                                MONSANTO COMPANY
                                      AND
                                  ECOGEN INC.



                          DATED AS OF JANUARY 24, 1996
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                              INVESTMENT AGREEMENT


         This Investment Agreement is made as of the 24th day of January, 1996
by and between MONSANTO COMPANY, a Delaware corporation with its general
offices at 800 North Lindbergh Blvd., St. Louis, Missouri 63167 ("Monsanto"),
and ECOGEN INC., a Delaware corporation with its general offices at 2005 Cabot
Boulevard West, Langhorne PA  19047 ("Ecogen").

                                    RECITALS

         Ecogen is engaged, among other things, in activities to
develop and commercialize certain valuable Bacillus thuringiensis ("Bt")
technology and owns or controls certain patents and patent applications related
thereto.

         1.      Pursuant to a Technology Assignment Agreement dated as of the
date hereof among Monsanto, Ecogen, and Ecogen-Bio Inc., a Delaware corporation
("Ecogen-Bio"), (the "Assignment Agreement"), Monsanto will purchase from
Ecogen and Ecogen-Bio certain technology and other rights relating to the Bt
technology of Ecogen and Ecogen-Bio including, without limitation, (a) certain
of Ecogen's Bt strain library, (b) certain of Ecogen's crystal protein gene
library, (c) certain of Ecogen's intellectual property rights regarding protein
engineering of Bt crystal proteins, including, without limitation, certain
patents and patent applications, (d) certain of Ecogen's intellectual property
rights associated with the Bt strains and genes being sold to Monsanto,
including, without limitation, certain patents and patent applications, and (e)
research records relating to the foregoing as set forth in the Assignment
Agreement.

         2.      Pursuant to a Research and Development Agreement dated as of
the date hereof between Monsanto and Ecogen (the "Research and Development
Agreement"), Ecogen will perform research and development on the Bt Technology.





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         3.      Monsanto desires to invest in Ecogen to provide capital to
fund (a) research and development on the Bt Technology and (b) other general
corporate purposes.

         NOW THEREFORE, in consideration of the promises and mutual covenants
contained herein, the parties agree as follows:

ARTICLE I.
                                  DEFINITIONS

         As used in this Agreement, the following terms shall have the meanings
specified below:

                 "Affiliate" with respect to a specified Person is another
Person that directly or indirectly through one or more intermediaries,
controls, or is controlled by, or is under common control with, the specified
Person.  For purposes of this definition, the term "control" (including the
terms "controlled by" and "under common control with") means the possession,
directly or indirectly, of the power to direct or cause the direction of the
management and policies of a Person, whether through the ownership of voting
securities, by contract or otherwise.

                 "Agreement" shall mean this Investment Agreement, as amended
from time to time.

                 "Annual Proxy Statement" has the meaning set forth in Section
3.3(c) hereof.

                 "Assignment Agreement" has the meaning set forth in the
Recitals hereto.

                 "Audited Financial Statements" has the meaning set forth in
Section 3.3(a) hereof.





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                 "Balance Sheet" shall mean the October 31, 1994 balance sheet
of Ecogen.

                 "Balance Sheet Date" shall mean October 31, 1994.

                 "Bt Technology" shall have the meaning set forth in the
Assignment Agreement.

                 "Business Day" shall mean any day of the week except Saturday,
Sunday and any legal holiday observed by a national banking association or one
of the parties.

                 "Closing" shall mean the closing of the transactions
contemplated by this Agreement.

                 "Commission" shall mean the Securities and Exchange
Commission, or any other federal agency at the time administering the
Securities Act.

                 "Common Stock" shall mean the shares of common stock, par
value $.01 of Ecogen.

                 "Control Securities" means securities of Ecogen, other than
Restricted Securities, owned by a Holder at the time such Holder would be
deemed to be an Affiliate of Ecogen.

                 "Corporate Approval" shall have the meaning set forth in
Section 3.2.

                 "Ecogen" shall mean Ecogen Inc., a Delaware corporation.

                 "Ecogen Securities" has the meaning set forth in Section 5.1
hereof.

                 "Exchange Act" shall mean the Securities Exchange Act of 1934,
as amended, or any similar Federal statute, and the rules and regulations of
the Commission issued under such Act, as they each may, from time to time, be
in effect.





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                 "Financial Statements" has the meaning set forth in Section
3.3(b) hereof.

                 "hereto", "hereunder", "herein", "hereof" and the like mean
and refer to this Agreement as a whole and not merely to the specific article,
section, paragraph or clause in which the respective word appears.

                 "Holder" means Monsanto and, subject to Section 7.8 hereof,
any subsequent holder of outstanding shares of the Common Stock purchased under
the terms of this Agreement.

                 "HSR" has the meaning set forth in Section 4.1(e) hereof.

                 "Indebtedness" of any Person shall mean all obligations of
such Person which in accordance with generally accepted accounting principles
are normally classified upon a balance sheet of such Person as liabilities of
such Person, and in any event shall include all (a) obligations of such Person
for borrowed money or which have been incurred in connection with the
acquisition of property, assets or services, (b) obligations secured by any
lien or other charge upon property or assets owned by such Person, even though
such Person has not assumed or become liable for the payment of such
obligations, (c) obligations created or arising under any conditional sale, or
other title retention agreement with respect to property where remedies of the
seller, lender or lessor under such agreement in the event of default are
limited to repossession or sale of property and (d) capitalized rentals.

                 "Indemnified Party" has the meaning set forth in Section
7.5(c) hereof.

                 "Indemnifying Party" has the meaning set forth in Section
7.5(c) hereof.

                 "Knowledge" with respect to Ecogen means the actual knowledge
of an officer of Ecogen.

                 "Monsanto" shall mean Monsanto Company, a Delaware
corporation.





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                 "Other Selling Stockholders" has the meaning set forth in
Section 7.1(c) hereof.

                 "Person" shall mean a corporation, association, partnership,
limited liability company, individual, trust, unincorporated organization, a
government agency or political subdivision thereof, or any other entity.

                 "Preliminary Prospectus" means a preliminary prospectus as
contemplated by Rule 430 or 430A under the Securities Act included at any time
in a Registration Statement.

                 "Pre-Offering Percentage" has the meaning set forth in Section
5.1 hereof.

                 "Price Per Share" has the meaning set forth in Section 2.2.

                 "Prospectus" means (i) a prospectus as first filed with the
Commission pursuant to Rule 424(b) under the Securities Act or, (ii) if no such
filing is required, the form of final prospectus included in a Registration
Statement at the effective date thereof or (iii) if a Term Sheet or Abbreviated
Term Sheet (as such terms are defined in Rule 434(b) and 434(c), respectively,
under the Securities Act) is filed with the Commission pursuant to Rule
424(b)(7) under the Securities Act, the Term Sheet or Abbreviated Term Sheet
and the last Preliminary Prospectus filed with the Commission prior to the time
the Registration Statement became effective, taken together (including, in each
case, the documents incorporated by reference therein pursuant to Item 12 of
Form S-3 under the Securities Act), together with any supplement to any of the
foregoing.

                 "Proxy Statements" has the meaning set forth in Section 3.3(e)
hereof.

                 "Purchase Price" has the meaning set forth in Section 2.2
hereof.





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                 "Purchased Shares" shall mean the shares of Common Stock of
Ecogen issued to Monsanto pursuant to this Agreement in an amount equal to the
nearest whole number derived by dividing (a) the Purchase Price by (b) the
Price Per Share.

                 "Register", "Registered" and "Registration", whether or not
capitalized, mean and refer to a registration effected by preparing and filing
a Registration Statement in compliance with the Securities Act and applicable
rules and regulations thereunder, and the declaration or ordering of the
effectiveness of such Registration Statement.

                 "Registration Statement" means any registration statement of
Ecogen filed under the Securities Act which covers any of the Registrable
Securities pursuant to the provisions of this Agreement, including the
Prospectus relating thereto and all amendments and supplements to such
registration statement, including post-effective amendments, all exhibits and
all material incorporated or deemed to be incorporated by reference in such
registration statement.

                 "Registrable Securities" means shares of Common Stock and all
such other securities of Ecogen acquired by Monsanto and any subsequent Holder.

                 "Registration Expenses" means all expenses incurred by Ecogen
in compliance with Article VII, including, without limitation, all registration
fees, qualification fees, filing fees, advertising and road show expenses
(excluding advertising and road show expenses incurred by a Holder), printing
expenses, escrow fees, fees and disbursements of counsel for Ecogen, blue sky
fees and expenses, and the expense of any special audits incident to or
required by any such registration (but excluding the compensation of regular
employees of Ecogen, which shall be paid in any event by Ecogen).

                 "Requesting Holder" means a Holder requesting any registration
pursuant to Section 7.1 hereof.





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                 "Research and Development Agreement" shall have the meaning
set forth in the Recitals hereto.

                 "Restricted Payments" shall mean dividends or other
distributions in respect of the stock of Ecogen or any Subsidiary (except
intercorporate dividends and dividends payable solely in stock of Ecogen) and
purchases, redemptions and other acquisitions, direct or indirect, of stock of
Ecogen or any Subsidiary.

                 "Restricted Securities" means the securities of Ecogen
acquired by a Holder from Ecogen or an Affiliate of Ecogen otherwise than
pursuant to a public offering.

                 "Reverse Stock Split" shall mean a one-for-five reverse stock
split of the outstanding shares of Common Stock of Ecogen which stock split is
the subject of a special proxy statement prepared in connection with a
stockholders' meeting called on January 29, 1996.

                 "SEC Documents" has the meaning set forth in Section 3.3
hereof.

                 "Securities Act" shall mean the Securities Act of 1933, as
amended, or any similar Federal statute, and the rules and regulations of the
Commission issued under such Act, as they each may, from time to time, be in
effect.

                 "Selling Expenses" means all underwriting discounts and
selling commissions applicable to the sale of Registrable Securities.

                 "Special Proxy Statement" has the meaning set forth in Section
3.3(e) hereof.

                 "Subsidiary" shall mean a subsidiary more than 50 percent of
whose outstanding securities representing the right, other than as affected by
events of default, to vote for the election of directors, is owned by the
applicable party and/or one or more of





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such party's other majority-owned Subsidiaries. Subsidiaries of Ecogen shall
also include the entities set forth in Schedule 3.1 hereto.

                 "13D Group" has the meaning set forth in Section 5.5 hereof.

                 "Unaudited Balance Sheet" has the meaning set forth in Section
3.3(d) hereof.

                 "Unaudited Financial Statements" has the meaning set forth in
Section 3.3(b) hereof.


                                  ARTICLE II.

                       PURCHASE AND SALE OF COMMON STOCK

         2.1     Common Stock.  Subject to the terms and conditions set forth
in this Agreement Ecogen will issue and sell, and Monsanto will purchase the
Purchased Shares.  On the Closing Date, Ecogen will issue and sell, and
Monsanto will purchase 3,450,000 of the Purchased Shares at the Price Per
Share.  From time to time after the Closing and subject to the provisions of
Section 2.4(b), Ecogen will issue and sell, and Monsanto will purchase, the
remainder of the Purchased Shares; provided that after July 31, 1996, Monsanto
shall have no further obligation to purchase any of the Purchased Shares then
unpurchased.

         2.2     Purchase Price.  The aggregate purchase price of the Purchased
Shares shall be $10,000,000 United States dollars (the "Purchase Price")
payable in increments with respect to the shares being purchased by Monsanto at
the Closing or on any subsequent date pursuant to Section 2.4(b) thereafter.
The Purchase Price shall be paid in immediately available funds by bank wire
transfer to the account designated by Ecogen.  The price per share shall be the
greater of (a) $2.12 or (b) 85% of the average of the last reported sales price
of Ecogen's common stock on the NASDAQ National Market on each of the last
sixty (60) trading days immediately preceding January 24, 1996 (the "Price Per
Share"), provided, however, in the event that the Closing shall occur after the
effective date of the Reverse Stock Split, the Price Per Share shall be
adjusted to reflect the Reverse Stock Split.  For





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purposes of this Section 2.2, a "trading day" shall mean a day that the NASDAQ
National Market is open for trading.  The aggregate number of Purchased Shares
shall be the number, rounded to the nearest whole number, determined by
dividing the Purchase Price by the Price Per Share.

         2.3     Stock Certificate.  At the Closing, and upon each subsequent
purchase thereafter, and upon payment of the Purchase Price with respect to
Purchased Shares then purchased, Ecogen shall deliver to Monsanto a stock
certificate or certificates in the name of Monsanto representing the number of
shares of Common Stock then purchased by Monsanto.

         2.4     Closing.

                          (a)     The Closing of this Agreement (the "Closing")
shall take place on January 24, 1996 at 10:00 a.m.  EST provided all of the
conditions to Closing stated in Article IV of this Agreement have been
satisfied or waived on or before such date; provided further, however, that
Monsanto and Ecogen each acknowledge that the Closing will occur prior to the
effective date of the Reverse Stock Split and that Ecogen will not have
sufficient authorized and unreserved shares of Common Stock to issue all of the
Purchased Shares.  Accordingly, at the Closing Ecogen will issue and sell, and
Monsanto will purchase, 3,450,000 of the Purchased Shares.  In the event that
all of the conditions to Closing stated in Article IV of this Agreement shall
not have been satisfied or waived on or before January 24, 1996, such Closing
shall take place as soon as practicable (but not later than the third Business
Day) after the satisfaction or waiver of such conditions and in no event later
than July 31, 1996. Monsanto and Ecogen shall use their best efforts to cause
the satisfaction of such conditions.  If such conditions of Closing have not
been satisfied or waived, and if Closing shall not have occurred, on or before
July 31, 1996, then this Agreement shall be terminated.  The Closing shall take
place at the offices of Ecogen or such other place as the parties may agree.

                          (b)     The remaining number of Purchased Shares not
purchased pursuant to Section 2.4(a) (the "Additional Shares") shall be issued
and sold by Ecogen and





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purchased and paid for by Monsanto immediately following the special
stockholders' meeting called on January 29, 1996, provided that the Reverse
Stock Split is approved by the stockholders of Ecogen at such meeting.  The
purchase price per share for such Additional Shares shall be the Per Share
Price determined pursuant to Section 2.2.  Monsanto's obligation to purchase
the Additional Shares shall be subject to the prior Closing of the transactions
contemplated under this Agreement pursuant to Section 2.4(a), to the delivery
by Ecogen to Monsanto of a certificate in the name of Monsanto for such
Additional Shares, and to the representations and warranties contained in
Sections 3.1 and 3.2 being true on such date as if made on such date or to
receipt of an opinion of counsel of Ecogen reasonably acceptable to Monsanto to
the effect that such Additional Shares are duly authorized and validly issued.
If the Reverse Stock Split is not approved by the stockholders of Ecogen, the
Additional Shares shall be issued and sold by Ecogen and purchased and paid for
by Monsanto from time to time, as shares become available or authorized for
issuance and upon at least five (5) days prior notice, in increments of at
least 50,000 shares, subject to the conditions set forth in the prior sentence;
provided, however, that Monsanto shall have no obligation to purchase any
Additional Shares after July 31, 1996.

                                  ARTICLE III.

                         REPRESENTATIONS AND WARRANTIES

         In order to induce Monsanto to purchase the Purchased Shares described
in Article II, Ecogen represents and warrants to Monsanto as follows:

         3.1     Organization, Subsidiaries.  Ecogen is a corporation duly
organized, validly existing and in good standing under the laws of the State of
Delaware and has all necessary corporate power and authority to carry on its
business as now conducted and as proposed to be conducted and to own or lease
and operate its properties.  Ecogen's only Subsidiaries are set forth on
Schedule 3.1 hereto.  Each Subsidiary is duly organized, validly existing and
in good standing under the laws of the jurisdiction of its incorporation and
has all necessary power and authority to carry on its business as now conducted
and as proposed to be conducted and to own, lease and operate its properties.
Except as set forth in Schedule





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3.1 hereof, Ecogen either directly, or indirectly through the Subsidiaries,
owns all of the issued and outstanding stock of each of its Subsidiaries free
and clear of all liens, claims, security interests or encumbrances.  Ecogen and
its Subsidiaries are duly licensed or qualified to do business and are in good
standing in every jurisdiction in which the nature of its business or the
ownership of their properties require such qualification except where such
failure to so qualify would not have a material adverse effect on the business
of Ecogen.  Except for cash equivalents and marketable securities held for
investment purposes, Ecogen  does not control, or have any contract or
commitment to own or control, any capital stock, bonds or other securities of,
and does not have a proprietary interest in, any Person other than the
Subsidiaries.  Ecogen and each of its Subsidiaries have delivered to Monsanto
complete and correct copies of their Certificates of Incorporation and By-Laws
as amended and in effect on the date hereof.

         3.2     Authorization and Binding Effect.  The execution and delivery
of this Agreement and the performance by Ecogen of its obligations hereunder
are within Ecogen's corporate power, have been duly authorized by proper
corporate action on the part of Ecogen, are not in violation of, or constitute
a default under any applicable existing law, rule or regulation of any
governmental agency or authority, any order or decision of any court, the
Certificate of Incorporation or By-Laws of Ecogen or the terms of any
agreement, restriction or undertaking to which Ecogen is a party or by which it
is bound, except violations or defaults which would not, individually or in the
aggregate, have a material adverse effect on Ecogen, and do not require the
approval or consent of the shareholders of Ecogen, any governmental body,
agency or authority or any other person or entity that has not been obtained;
except (i) the filing of any notice subsequent to Closing which may be required
by applicable state or federal securities laws and (ii) stockholder approval
required for the Reverse Stock Split or other corporate action necessary to
increase the number of shares of Common Stock available for issuance under the
Agreement if the Reverse Stock Split is not approved ("Corporate Approval").





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         3.3     Consolidated Financial Statements: Other Information.  Ecogen
has previously delivered to Monsanto true, accurate and complete copies of the
following documents, including, the exhibits and schedules thereto:

                          (a)     Annual Report on Form 10-K for the transition
period ended October 31, 1994, as filed with the Commission, which contains the
audited consolidated balance sheets of Ecogen as of December 31, 1993, and
October 31, 1994, and audited consolidated statements of operations, cash flows
and stockholders' equity for the years ended December 31, 1992 and 1993 and the
ten (10) months ended October 31, 1994, certified by KPMG Peat Marwick LLP and
heretofore furnished to Monsanto (together the "Audited Financial Statements");

                          (b)     Quarterly Reports on Form 10-Q for the three
months ended January 31, 1995, April 30, 1995 and July 31, 1995, as filed with
the Commission (the "Unaudited Financial Statements" and, together with the
Audited Financial Statements, the "Financial Statements");

                          (c)     Proxy statement in definitive form for its
1995 annual meeting of stockholders as filed with the Commission (the "Annual
Proxy Statement");

                          (d)     Unaudited Consolidated Balance Sheet, dated
October 31, 1995 (the "Unaudited Balance Sheet") attached hereto as Exhibit
3.3(d); and

                          (e)     Proxy Statement in definitive form for a
Special Meeting of Shareholders scheduled for January 29, 1996 (the "Special
Proxy Statement" and, together with the Annual Proxy Statement, the "Proxy
Statements").

         Except for Current Reports on Form 8-K dated November 7, 1995 and
November 24, 1995 and the Special Proxy Statement dated December 18, 1995,
Ecogen has filed no other reports or registration statements with the
Commission since July 31, 1995. Since January 1, 1994, Ecogen has filed all
reports, schedules, forms, statements and other documents





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required to be filed by it with the Commission pursuant to the requirements of
the Exchange Act and the Securities Act, including, without limitation, all of
the documents listed in paragraphs (a), (b), (c) and (e) above (all of the
foregoing filed prior to the date hereof being hereinafter referred to as the
"SEC Documents").  Ecogen has delivered to Monsanto true and complete copies of
the SEC Documents.  As of their respective dates, the SEC Documents (as the
same may have been amended) complied in all material respects with the
requirements of the Exchange Act and the Securities Act and the rules and
regulations of the Commission promulgated thereunder applicable to such SEC
Documents, and, as of their respective dates, none of the SEC Documents (when
read together with all exhibits included therein and financial statement
schedules thereto and documents incorporated by reference) contained any untrue
statement of a material fact or omitted to state a material fact required to be
stated therein or necessary in order to make the statements therein, in light
of the circumstances under which they were made, not misleading.  The Financial
Statements were prepared in accordance with generally accepted accounting
principles consistently applied throughout the periods involved except as may
be otherwise indicated in such financial statements or the notes thereto, and
are correct and complete and fairly present in all material respects the
consolidated financial condition of Ecogen and its Subsidiaries, as of those
dates and the results of their operations for the periods then ended.

         3.4     Litigation.      Except as set forth in Ecogen's October 31,
1994 Annual Report on Form 10-K and in Ecogen's Quarterly Reports on Form 10-Q
provided to Monsanto pursuant to Section 3.3(b) and except as disclosed on
Schedule 3.4 hereto, there are no judicial or administrative actions, suits,
proceedings or investigations pending or, to the best of Ecogen's knowledge,
threatened against Ecogen or any of its Subsidiaries or which question the
validity of this Agreement or of any action taken or to be taken pursuant to or
in connection with the provisions of this Agreement, and to the best of
Ecogen's knowledge, no facts exist which would constitute a basis for any such
action, suit, proceeding or investigation, which if concluded adversely against
Ecogen would have a materially adverse effect on the business or financial
condition of Ecogen and its Subsidiaries taken as a whole.





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         3.5      Material Contracts.  The contracts listed as Exhibits
10.1 - 10.119 on the list of exhibits under Part IV, Item 14(a)(3) of the
October 31, 1994 Form 10-K, and filed (or incorporated by reference) as
Exhibits 10.1 - 10.119 to the October 31, 1994 Form 10-K, were all of the
contracts required to be listed pursuant to the Commission's instructions for
the completion of Form 10-K and filed as exhibits pursuant to Item 601(b)(10)
of the Commission's Regulation S-K.  Except as disclosed in Ecogen's January
31, 1995, April 30, 1995 or July 31, 1995 Form 10-Q and in Ecogen's Current
Reports on Form 8-K dated November 7, 1995 and November 24, 1995, neither
Ecogen nor any of its Subsidiaries has entered into any contract since October
31, 1994 which is required to be filed as an exhibit to a quarterly report on
Form 10-Q or an annual report on Form 10-K. All of those contracts filed as
exhibits to the October 31, 1994 Form 10-K or disclosed in Ecogen's January 31,
1995, April 30, 1995 or July 31, 1995 Form 10-Q or in Ecogen's Current Reports
on Form 8-K dated November 7, 1995 and November 24, 1995 which are material to
the business and operations of Ecogen as presently conducted are in full force
and effect and, to the best of Ecogen's knowledge, all parties to such
contracts have performed in all material respects all obligations required to
be performed by them to date, are not in default and do not have a right of
rescission.  Neither Ecogen nor any of its Subsidiaries has any outstanding
powers of attorney, except routine powers of attorney relating to
representation before governmental agencies or given in connection with
qualification to conduct business in another jurisdiction.  No employees of
Ecogen or any of its Subsidiaries are covered by any collective bargaining
agreement.

         3.6     Taxes.  Ecogen and each of its Subsidiaries has filed when due
all federal, state and local income and other tax returns, reports and
declarations which are required to have been filed by it and has, to the best
of Ecogen's knowledge, paid all taxes which have become due pursuant thereto
and all other taxes, assessments and other governmental charges imposed by law
upon it or any of its properties, assets, income, receipts, payrolls,
transactions, capital, net worth or franchises other than those not delinquent
except where the failure to file or pay would not have a material adverse
effect on the business of Ecogen.  All such tax returns are complete and
correct in all material respects as filed.  To the best of Ecogen's knowledge,
there is no tax lien upon any property or asset





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of Ecogen, whether owned or leased, except for liens for taxes not yet payable
and liens which do not have a material adverse effect on the business of
Ecogen.  To the extent that tax liabilities have accrued on or before the
Balance Sheet Date but have not become payable, to the best of Ecogen's
knowledge, they have been adequately reflected as liabilities on the Balance
Sheet and adequate provision for payment thereof has been made.  There have
been no examinations by any state or federal taxing authority of the books and
financial statements of Ecogen or any of its Subsidiaries.  The accruals and
reserves for federal, state or local taxes, as shown on the Balance Sheet are
adequate in the opinion of Ecogen. Neither Ecogen nor any of its Subsidiaries
has granted any waiver of any statute of limitation with respect to, or any
extension of a period for the assessment of, any federal, state or local tax.
To the best of Ecogen's knowledge, Ecogen has withheld and duly paid to the
appropriate governmental authority all taxes required to be withheld by it
pursuant to any ordinance, statute or other law except where the failure to
withhold or pay would not have a material adverse effect on the business of
Ecogen.

         3.7     Capitalization.  Prior to the Reverse Stock Split, the
authorized capital stock of Ecogen consists of (i) 42,000,000 shares of common
stock, par value $.01 per share, of which 29,996,287 shares were issued and
outstanding at December 8, 1995, and no additional shares have subsequently
been issued except upon exercise of options, warrants or convertible securities
outstanding as of December 8, 1995; and (ii) 7,500,000 shares of preferred
stock, par value $.01 per share, issuable in series, of which 350,000 shares
have been designated Series B convertible preferred stock and 23,334 shares of
which are issued and outstanding at December 8, 1995, and of which 160,000
shares have been designated Series C Convertible Preferred Stock, and 122,000
shares of which are issued and outstanding as of December 8, 1995.  After the
consummation of the reverse stock split described in the Special Proxy
Statement, the authorized capital stock of Ecogen will consist of (i)
42,000,000 shares of common stock, par value $.01 per share of which
approximately 5,990,000 shall be issued and outstanding; and (ii) 7,500,000
shares of preferred stock, par value $.01 per share with no change in the
designated preferred or the issued and outstanding shares of preferred stock.
The issued and outstanding shares of Ecogen have been duly and validly issued
and are fully paid and nonassessable, and the Purchased Shares, when issued,





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will be duly and validly issued and fully paid and nonassessable.  Except for
180,778 shares of common stock, Ecogen holds no shares of its common stock or
preferred stock in its treasury.  Except as disclosed in Schedule 3.7, there
are no outstanding options, warrants or other rights in existence, other than
under this Agreement and under the Warrant Agreement dated as of November 2,
1995 between Monsanto and Ecogen, to acquire from Ecogen any of its shares of
capital stock.  Since the Balance Sheet Date there have been no dividends,
Restricted Payments or other distributions declared or paid in respect of the
shares of capital stock of Ecogen.

         3.8     Compliance With Instruments, Laws; Governmental
Authorizations.  Neither Ecogen nor any of its Subsidiaries is (i) in violation
of any term or provision of its Certificate of Incorporation or By-Laws or, to
the best of Ecogen's knowledge, any governmental license or permit; (ii) in
violation or default under any material contract, agreement or other
instrument; or (iii) in violation of any statute, law, ordinance, rule,
regulation, judgment, order, decree, permit, concession, grant, franchise,
license or other governmental authorization or approval which is material to
its business or operations.  All material permits, concessions, grants,
franchises, licenses and other governmental authorizations and approvals
necessary for the conduct of the business of Ecogen and its Subsidiaries have
been duly obtained and are in full force and effect, and there are no
proceedings pending or, to the best of Ecogen's knowledge, threatened which may
result in the revocation, cancellation or suspension, or any adverse
modification, of any permit, concession, grant, franchise, license or other
governmental authorization which would have a material adverse effect on the
business or financial condition of Ecogen and its Subsidiaries taken on a
whole.  There have been no citations, fines or penalties heretofore assessed
against Ecogen or any of its Subsidiaries and, to the best of Ecogen's
knowledge, Ecogen and each of its Subsidiaries have complied with in all
material respects any and all federal, state or local laws, including but not
limited to laws relating to air or water pollution, solid waste disposal or
other environmental protection matters, or relating to occupation, health or
safety, and no such citations, fines or penalties have been assessed or, to the
best of Ecogen's knowledge, have been threatened since the Balance Sheet Date
or are now being threatened.  There exists no default nor has any act or
omission occurred which, with the





                                       16
<PAGE>   18
giving of notice or the passage of time, would constitute a default under the
provisions of any instrument evidencing Indebtedness or liability or any
agreement relating thereto or any other agreement or instrument to which Ecogen
is a party, and which would have a material adverse effect on the business or
financial condition of Ecogen.

         3.9     Patents and Technology Rights.

                 (a)      Except as set forth in Schedule 3.9 hereof and except
with respect to bioinsecticide product supply, manufacturing, distributor and
marketing agreements to which Ecogen is a party, to the best of Ecogen's
Knowledge, Ecogen and its Subsidiaries and their successors and assigns have a
right (1) to practice all processes and process steps as now practiced or
planned to be practiced by Ecogen and its Subsidiaries, and (2) to make, use
and sell throughout the world, products now made, used or sold or planned to be
made, used or sold by Ecogen and its Subsidiaries without substantial risk of
infringement of any right, interest, or patent of any third party.

                 (b)      Ecogen and each of its Subsidiaries owns and
possesses, or is licensed or is otherwise entitled to use, all patents,
trademarks, copyrights and such pending applications therefor and trade
secrets, technologies, know-how, processes and other proprietary rights
necessary for conducting its business operations; provided, however, that
certain intellectual property rights will be sold by Ecogen to Monsanto
pursuant to the Assignment Agreement.

                 (c)      Except as set forth in Schedule 3.9 hereof, to the
best of Ecogen's Knowledge, there exists no contest, litigation, infringement,
fraud, misappropriation, or misuse by Ecogen, pending or threatened in respect
of, any patent or application therefor, or any license or agreement of a third
party which is necessary for the conduct of Ecogen's business or the business
of any of its Subsidiaries.

         3.10    Certain Transactions.  No Affiliate of Ecogen that is not a
Subsidiary of Ecogen owns or controls, directly or indirectly, in whole or in
part, any property, asset or





                                       17
<PAGE>   19
right, tangible or intangible (including but not limited to, any patent,
trademark, service mark, trade name, brand name, copyright, or pending
application for any patent, trademark, service mark, or copyright, invention,
process, know-how, formula, design or trade secret), which Ecogen or any of its
Subsidiaries is presently operating or using or the use of which is necessary
for its business.

         3.11    Absence of Changes.   Since the Balance Sheet Date, except as
disclosed on Ecogen's January 31, 1995, April 30, 1995 and July 31, 1995 Form
10-Qs, the Unaudited Balance Sheet, the Current Report on Form 8-K dated
November 7, 1995, the Current Report on Form 8-K dated November 29, 1995, or as
specified in Schedule 3.11 hereto, Ecogen and its Subsidiaries have not:

                 (a)      undergone any changes in its condition, properties,
assets, liabilities, business or operations which are in the aggregate
materially adverse except for net losses of $20,834,022 for the nine months
ended July 31, 1995 and a continuation of operating losses and a liquidity
position similar to that reported in the July 31, 1995 Form 10-Q;

                 (b)      declared, set aside, made or paid any Restricted
Payment;

                 (c)      incurred any Indebtedness or issued or sold any debt
securities except for Indebtedness incurred in connection with the purchase or
lease of property in the ordinary course of business which in the aggregate is
not materially adverse to Ecogen and Indebtedness to Monsanto;

                 (d)      mortgaged, pledged, licensed, sublicensed or
subjected to any lien, lease, security interest or other charge or encumbrance
any of its properties or assets, tangible or intangible which in the aggregate
materially adversely affect the financial condition, property, assets,
business, or operations of Ecogen and its Subsidiaries taken as a whole, other
than liens and security interests in favor of Monsanto;





                                       18
<PAGE>   20
                 (e)      acquired or disposed of any assets or properties
(except for common stock of Ecogen) in any transaction with any officer or
director or with any shareholder owning more than 5% of the outstanding common
stock except for transactions that have a value of less than $60,000;

                 (f)      forgiven or cancelled any debts or claims, or waived
any rights having a value of more than $60,000 except in the ordinary course of
business which in the aggregate is not materially adverse to Ecogen;

                 (g)      suffered any damage, destruction or loss (whether or
not covered by insurance) which in the aggregate materially adversely affects
the financial condition, properties, assets, business or operations of Ecogen
and its Subsidiaries; or

                 (h)      incurred other than in the ordinary course of
business any liability or obligation (whether absolute, accrued, contingent or
otherwise), exceeding $1,000,000 other than liabilities and obligations to
Monsanto.

         3.12    Restrictions on Personnel.   To the best of Ecogen's
Knowledge, no officer or employee of Ecogen or any of its Subsidiaries has
entered into any agreement which is now in effect with any Person other than
Ecogen or its Subsidiaries (a) requiring such officer or employee to assign any
interest in any invention or trade secrets developed while employed by Ecogen
or any of its Subsidiaries or (b) containing any prohibition or restriction of
competition or solicitation of customers which if enforced against such officer
or employee would have a material adverse effect on the business or financial
condition of Ecogen or any of its Subsidiaries.

         3.13    Certain Payments.  Neither the Internal Revenue Service, the
Commission nor any other federal, state, local or foreign government agency or
entity has initiated or, to the best of Ecogen's knowledge, threatened any
investigation of any payments made by Ecogen and alleged to have been of the
type covered by Section 162(c) of the Internal Revenue Code.





                                       19
<PAGE>   21
         3.14    Disclosure.  Neither this Agreement nor any exhibit or
schedule hereto, nor any statement, list or certificate delivered to Monsanto
pursuant to this Agreement contains any untrue statement of a material fact or
omits to state a material fact necessary in order to make the statements
contained herein and therein in the context in which they were made not
misleading.

         3.15    Brokers.  This Agreement was not induced or incurred through
any person, firm or corporation acting as a broker or finder, other than Len
Bogner, for whose fees Ecogen shall be responsible.  Ecogen agrees to hold
Monsanto harmless from any loss, damage, cost or expense resulting from any
claim by any person, firm or corporation based upon any such person, firm or
corporation having acted as a broker or finder for Ecogen or any other Person
in connection with the transactions contemplated by this Agreement, the
Assignment Agreement and/or the Research and Development Agreement.

                                  ARTICLE IV.

           CONDITIONS PRECEDENT TO PURCHASE AND SALE OF COMMON STOCK

         4.1     Conditions to Monsanto Obligations.  The obligation of
Monsanto to purchase the Purchased Shares described in Article II is subject to
the satisfaction of the following conditions each of which may be waived in
writing by an authorized representative of Monsanto:

                 (a)      Opinion of Counsel.  Monsanto shall have received an
opinion addressed to Monsanto and dated as of the Closing, of Dechert, Price &
Rhoads, counsel for Ecogen, satisfactory in substance and form to Monsanto and
its counsel and substantially in the form attached hereto as Exhibit 4.1(a). In
rendering such opinion, counsel may rely to the extent deemed appropriate on
the certificates of officers or employees of Ecogen and of public officials as
to matters of fact and authenticity of documents and on opinions of counsel in
other states as to questions under the laws of such states.





                                       20
<PAGE>   22
                 (b)      Representations and Warranties True and Correct. The
representations and warranties in Article III hereof shall be true and correct
on the date of Closing.  Monsanto shall have received from the President of
Ecogen an officer's certificate to the foregoing effect dated as of the
Closing.

                 (c)      Closing Certificates.  Monsanto shall have received
copies, certified by the Secretary or Assistant Secretary of Ecogen to be true
and correct and in full force and effect, of (i) the Certificate of
Incorporation and By-Laws of Ecogen; (ii) resolutions of the Board of Directors
of Ecogen authorizing the issuance of the Purchased Shares, the execution and
delivery of this Agreement and any other documents to be executed by or on
behalf of Ecogen pursuant to this Agreement; and (iii) a statement containing
the names and titles of the officer or officers of Ecogen authorized to sign
this Agreement and other documents required by this Agreement, together with
true signatures of such officers.

                 (d)      Proceedings Satisfactory to Monsanto.   All
proceedings taken in connection with the transaction contemplated by this
Agreement and all instruments, authorizations and other documents applicable
thereto shall be reasonably satisfactory in form and content to Monsanto and
Monsanto shall have received copies of all such documents reasonably required
by it.

                 (e)      Approvals.  The pre-transaction filing and waiting
period requirements applicable to the transactions contemplated by this
Agreement, the Assignment Agreement and the Research and Development Agreement
under the Hart-Scott-Rodino Antitrust Improvement Act of 1976, as amended
("HSR"), shall have expired or shall have been terminated, and any necessary
foreign approvals shall have been obtained, and there shall not be pending or
threatened any governmental litigation or proceeding which restrains, prohibits
or prevents or in the reasonable opinion of counsel presents a significant risk
of restraining, prohibiting or preventing, or changing the terms of, or
obtaining material damages in connection with, the transactions contemplated by
this Agreement, the Assignment Agreement or the Research and Development
Agreement.





                                       21
<PAGE>   23
                 (f)      Research and Development Agreement.  Ecogen shall
have executed and delivered the Research and Development Agreement.

                 (g)      Assignment Agreement.  The transactions contemplated
under the Assignment Agreement shall have been closed simultaneously with the
Closing hereunder.

         4.2     Conditions to Ecogen's Obligation.  The obligation of Ecogen
to sell the Purchased Shares described in Article II is subject to the
satisfaction of the following conditions each of which may be waived in writing
by an authorized representative of Ecogen.

                 (a)      At Closing, Monsanto shall have delivered the
Purchase Price with respect to those Shares purchased at Closing.

                 (b)      Closing Certificate.  Ecogen shall have received a
Certificate, dated as of the Closing Date and executed by an officer of
Monsanto to the effect that the representations and warranties of Monsanto
contained in this Agreement are true and correct in all respects on and as of
the Closing as though made on and as of such date.

                 (c)      Representations, Warranties and Covenants.

                          (i)     All representations and warranties of
         Monsanto made in this Agreement, or in any Exhibit or certificate
         delivered pursuant hereto shall be true and complete in all material
         respects on and as of the Closing Date with the same force and effect
         as if made on and as of that date.

                          (ii)    All terms, covenants, and conditions to be
         complied with and performed by Monsanto on or prior to the Closing
         Date shall have been complied with or performed in all material
         respects.





                                       22
<PAGE>   24
                 (d)      Adverse Proceedings.  No suit, action or governmental
proceeding shall have been instituted or threatened against and no order,
decree, or judgment of any court, agency, or other governmental authority shall
have been rendered against Monsanto which it is reasonably believed would
render it unlawful, as of the Closing Date, to effect the transactions
contemplated by this Agreement in accordance with its terms.

                 (e)      HSR Act Approval.  The applicable HSR Waiting Period
shall have expired or shall have been terminated and any necessary foreign
approvals shall have been obtained and there shall not be pending or threatened
any governmental litigation proceeding which restrains, prohibits, or prevents
or in the reasonable opinion of counsel presents a significant risk of
restraining, prohibiting or preventing, or changing the terms of, or obtaining
material damages in connection with the transactions contemplated by this
Agreement, the Assignment Agreement or the Research and Development Agreement.

                 (f)      Research and Development Agreement.  Monsanto shall
have executed and delivered the Research and Development Agreement.

                 (g)      Assignment Agreement.  The transactions contemplated
under the Assignment Agreement shall have been closed simultaneously with the
Closing hereunder.

                 (h)      Senior Promissory Note.  Monsanto shall have
delivered to Ecogen, marked paid, the original of the Senior Promissory Note
dated November 2, 1995 in the face amount of $2,000,000 and payable to
Monsanto.

                 (i)      Security Agreement.  Monsanto shall have executed and
delivered such documents required to terminate the security interest granted
pursuant to the Security Agreement dated as of November 2, 1995 by and between
Monsanto and Ecogen.

                 (j)      Warrant Agreement.  Monsanto shall have delivered to
Ecogen, marked cancelled, the original of the Warrant to Purchase Two Million
Shares of Common





                                       23
<PAGE>   25
Stock dated November 2, 1995 granted pursuant to the Warrant Agreement by and
between Monsanto and Ecogen dated November 2, 1995.

                                   ARTICLE V.

              RIGHT OF FIRST REFUSAL AND LIMITATIONS ON OWNERSHIP

         5.1     Right of First Refusal.  If during the term of the Research
and Development Agreement and for 2 years following its termination Ecogen
agrees to sell shares of its common stock or other voting securities ("Ecogen
Securities") in a private or public offering other than (i) Ecogen Securities
issued to Ecogen employees or directors pursuant to Ecogen's stock option
plans, or to members of Ecogen's Scientific Advisory Board, or issued in
connection with any stock options, warrants or other rights in existence as of
the Closing Date, (ii) upon the conversion or exchange of convertible or
exchangeable securities outstanding as of the date of this Agreement, (iii) an
equity investment in Ecogen by the other party to, or partner in, any license
agreement, research and development agreement, distribution agreement,
joint-venture or other similar corporate alliance, provided that such equity
investment is made as a condition to Ecogen's good faith agreement to enter
into such license agreement, research and development agreement, distribution
agreement, joint-venture or other similar corporate alliance, (iv) in
connection with a merger or acquisition, or (v) to any underwriter, placement
agent or financial advisor in connection with any financing or other
transaction by Ecogen, and provided Monsanto still owns all of the Purchased
Shares at the time of such private or public offering, Monsanto shall have the
right, but not the obligation, to acquire upon the same terms and conditions
applicable to such private or public offering all or any portion of such Ecogen
Securities sufficient for Monsanto to maintain after the offering the same
percentage of ownership of issued and outstanding Ecogen Securities that
Monsanto possessed immediately prior to the offering (the "Pre-Offering
Percentage").

         5.2     Private Offering.  With respect to a private offering, Ecogen
shall no later than five (5) Business Days after the execution of any agreement
entered into in connection with such private offering notify Monsanto in
writing of the proposed offering (a





                                       24
<PAGE>   26
"Notice") which Notice shall include the description of the securities proposed
to be issued by Ecogen and specify the number, price and payment and other
terms, together with any letter of intent and the final contract, if available.
Monsanto shall have twenty (20) Business Days from the date of receipt of
Ecogen's notice in which to advise Ecogen whether Monsanto elects to exercise
its rights under Section 5.1.  If Monsanto does not respond, or if Monsanto
indicates that it will not exercise its rights, Monsanto shall be considered
irrevocably to have waived its rights under Section 5.1 with respect to the
private offering.  If Monsanto timely advises Ecogen that Monsanto will
exercise its rights under Section 5.1, Monsanto shall have the right to acquire
all or any portion of the necessary amount of the Ecogen Securities to maintain
Monsanto's Pre-Offering Percentage at the price and upon the terms (other than
any registration rights which are on terms different than those granted
hereunder) specified in the Notice.  Closing shall be in accordance with the
terms of the private offering agreement, provided, however, that if such
agreement provides for the payment of consideration other than cash, Monsanto
shall have the right to purchase its Pre-Offering Percentage of shares for the
cash equivalent amount of such other consideration.  Monsanto shall make such
investment representations to Ecogen and shall provide Ecogen with such other
documentation at closing as is reasonably required by Ecogen to comply with
applicable securities laws.  The cash equivalent amount referred to above shall
be determined by an independent third party reasonably agreed to by Monsanto
and Ecogen (the costs and expenses of which shall be paid by Ecogen).

         5.3     Public Offering.  With respect to a public offering, Ecogen
shall notify Monsanto no later than five (5) Business Days after Ecogen has
entered into a letter of intent with its underwriters, and shall provide
Monsanto with a copy of any letter of intent with its underwriters.  Monsanto
shall have twenty (20) Business Days from the date of receipt of Ecogen's
notice in which to advise Ecogen whether Monsanto elects to exercise its rights
under Section 5.1.  If Monsanto does not respond or if Monsanto indicates that
it will not exercise its rights, Monsanto shall be considered irrevocably to
have waived its rights under Section 5.1 with respect to the public offering.
If Monsanto timely advises Ecogen that Monsanto desires to retain its rights
under Section 5.1, then when Ecogen files a registration statement containing a
preliminary prospectus with the Commission, Ecogen shall provide





                                       25
<PAGE>   27
Monsanto with copies of the preliminary prospectus and all subsequent
amendments.  Monsanto shall have twenty (20) Business Days from its receipt of
the preliminary prospectus in which to exercise its rights under Section 5.1 by
making an offer to acquire all or any portion of the necessary amount of Ecogen
Securities to maintain Monsanto's Pre-Offering Percentage based on the price
and the other terms contained in the final prospectus, except that the price to
be paid by Monsanto shall be net of any underwriting discount and selling
commissions.  No such offer to buy shall be accepted prior to the time that the
registration statement becomes effective.  The registration statement shall
indicate that Monsanto has anti-dilution rights to purchase Ecogen Securities
on the terms offered to the public.

         5.4     Limitations.  Notwithstanding the preceding provisions of this
Article V, Ecogen shall not be required to issue any fractional shares as a
result of Monsanto's exercise of its rights under Section 5.1. Ecogen shall not
be required to transfer any Ecogen Securities to Monsanto under this Article V,
if to do so would result in the violation of any applicable law, rule or
regulation.

         5.5     Limitations on Monsanto's Ownership.  Except for purchases of
Ecogen Securities made in accordance with this Article V, prior to the third
anniversary of the Closing, neither Monsanto nor any of its Affiliates shall
directly or indirectly acquire any Ecogen Securities (except by way of stock
dividends, stock splits, or similar events affecting holders of Ecogen
Securities generally) if the effect of the acquisition would be to increase the
aggregate voting power in the election of directors of all Ecogen Securities
owned by Monsanto and its Affiliates to more than twenty-five percent (25%) of
the total combined voting power of all Ecogen Securities then outstanding;
provided, however, that nothing contained in this Agreement shall be deemed to
prohibit Monsanto and its Affiliates from acquiring Ecogen Securities (through
open market purchases or otherwise) so long as the effect of such acquisitions
does not increase the aggregate voting power in the election of directors of
all Ecogen Securities owned by Monsanto and its Affiliates to more than
twenty-five percent (25%) of the total combined voting power of all Ecogen
Securities then outstanding.  Notwithstanding the foregoing limitation:





                                       26
<PAGE>   28
                 (a)      Monsanto or any of its Affiliates may acquire an
unlimited percentage of Ecogen Securities prior to the third anniversary of the
Closing if any of the following events occur:

                          (i)     A tender or exchange offer is made by any
         Person or 13D Group (other than an Affiliate of or Person acting in
         concert with Monsanto) to acquire Ecogen Securities and/or other
         securities of Ecogen that, if added to the Ecogen Securities and other
         securities of Ecogen already owned by that Person or 13D Group, would
         represent more than thirty percent (30%) of the total combined voting
         power or profit and loss participation of all Ecogen Securities and
         other securities of Ecogen issued and outstanding;

                          (ii)    There is public disclosure that Ecogen
         Securities and/or other securities of Ecogen representing more
         than thirty percent (30%) of the total combined voting power
         or profit and loss participation of all Ecogen Securities and
         other securities of Ecogen issued and outstanding have been
         acquired or are proposed to be acquired by any Person or 13D
         Group (other than an Affiliate of or Person acting in concert
         with Monsanto);

                          (iii)   Any Person or 13D Group (other than an
         Affiliate of or Person acting in concert with Monsanto) shall
         beneficially own Ecogen Securities representing more than thirty
         percent (30%) of the total combined voting power of all issued and
         outstanding Ecogen Securities;

                          (iv)    At any election or series of elections,
         persons not proposed for nomination or nominated by the management of
         Ecogen are elected as directors of Ecogen, and together constitute
         fifty percent (50%) or more of Ecogen's Board of Directors; or

                          (v)     Any Person or 13D Group (other than an
         Affiliate of or Person acting in concert with Monsanto) solicits or
         receives valid proxies for the





                                       27
<PAGE>   29
         election of Ecogen's Board of Directors representing an aggregate of
         thirty percent (30%) or more of the total combined voting power of all
         Ecogen Securities issued and outstanding.

                 (b)      Neither Monsanto nor any of its Affiliates shall be
required to dispose of any shares of Ecogen Securities if their aggregate
percentage ownership is increased as a result of Ecogen's recapitalization or
any other action taken by Ecogen.

As used herein, "13D Group" means any group of persons formed for the purpose
of acquiring, holding, voting or disposing of securities that would be required
under Section 13(d) of the Exchange Act and the related rules and regulations
(as now in effect, and based on present legal interpretations) to file a
statement on Schedule 13D or 13G with the Commission as a "person" within the
meaning of Section 13(d)(3) of the Exchange Act if the group beneficially owned
Ecogen Securities representing more than five percent (5%) of the total
combined voting power of all Ecogen Securities then issued and outstanding.

                                  ARTICLE VI.

                             LIMITATION ON TRANSFER

         6.1     Limitation on Transfer.  During the term of the Research and
Development Agreement, Monsanto shall not directly or indirectly offer for
sale, or transfer any Common Stock acquired by Monsanto pursuant to this
Agreement.  Notwithstanding the foregoing, the limitations on transfers set
forth in this Section 6.1 shall terminate upon the occurrence of an event set
forth in Section 5.5(a).





                                       28
<PAGE>   30
                                  ARTICLE VII.

                              REGISTRATION RIGHTS

         7.1     Requested Registration.

                 (a)      Request for Registration.  Subject to Section 6.1,
Holders of Registrable Securities shall have the right to request (with such
requests in writing and stating the number of shares of Registrable Securities
to be disposed of and the intended method of disposition of shares by such
Holders) up to two (2) registrations on Form S-3 at Ecogen's expense and an
unlimited number of additional registrations on Form S-3 at the selling
Holder's expense, provided that a request for registration includes at least
fifty percent (50%) of the aggregate Registrable Securities held on such date
and (ii) the requests for additional registrations are made by Holders of at
least twenty percent (20%) of the Registrable Securities, and (iii) such
Registerable Securities have an aggregate offering price of at least $1,000,000
or represent all of the Registerable Securities of the Holders held on such
date, subject only to the following:

                          (i)     Ecogen shall not be required to effect a
         registration pursuant to this Section 7.1 prior to the second
         anniversary of the Closing unless Monsanto otherwise has a right
         hereunder to sell Registrable Securities before such date.

                          (ii)    Ecogen shall not be required to effect a
         registration pursuant to this Section 7.1 within one hundred eighty
         (180) days after the effective date of the last such registration
         pursuant to this Section 7.1.

                          (iii)   Ecogen shall not be required to effect a
         registration in any particular jurisdiction in which Ecogen would be
         required to execute a general consent to service of process in
         effecting such registration, qualification or compliance, unless
         Ecogen is already subject to service in such jurisdiction and except
         as may be required by the Securities Act or applicable rules or
         regulations thereunder.





                                       29
<PAGE>   31
                          (iv)    Ecogen shall not be required to effect a
         registration for a period of not more than ninety (90) days
         immediately following the delivery of a certificate signed by the
         President of Ecogen to the Requesting Holders stating that, in the
         good-faith judgment of the Board of Directors of Ecogen, it would be
         seriously detrimental to Ecogen and its shareholders for a
         Registration Statement to be filed on or before the date filing would
         otherwise be required hereunder; provided, however, that Ecogen may
         not utilize this right more than once in any twelve (12) month period
         and Ecogen may not exercise this right based on the fact that Ecogen
         has recently registered any of its securities for the account of
         another security holder or holders exercising their respective demand
         registration rights.

         If Ecogen cannot qualify for registration on Form S-3, then Ecogen
         shall effect any registration required or requested by the Holder on
         Form S-1, or such other appropriate form, in which event this Section
         7.1 shall apply in all respects as if the words "Form S-3" were
         replaced by the words "Form S-1" or the appropriate designation for
         such other form.

                 (b)      Notice of Inclusion.  Ecogen shall give written
notice to all Holders of Registrable Securities of the receipt of a request for
registration pursuant to this Section 7.1 and shall provide a reasonable
opportunity for other Holders to participate in the registration; provided,
however, that, if the registration is for an underwritten offering, then the
terms of Section 7.1(c) hereof shall apply to all participants in such
offering.  Subject to the foregoing, Ecogen shall use its best efforts to
effect promptly the registration of all shares of Registrable Securities on
Form S-3 to the extent requested by the Holder or Holders thereof for purposes
of disposition.

                 (c)      Underwriting.  If the Requesting Holders intend to
distribute the Registrable Securities covered by their request by means of an
underwriting, then they shall so advise Ecogen as a part of their request made
pursuant to this Section 7.1 and Ecogen shall include such information in the
written notice referred to in Section 7.1(b) hereof.  The right of any Holder
to registration pursuant to this Section 7.1 shall be conditioned upon such





                                       30
<PAGE>   32
Holder's participation in such underwriting and the inclusion of such Holder's
Registrable Securities in the underwriting to the extent requested and to the
extent provided herein.

                 Ecogen shall (together with all Holders proposing to
distribute their securities through such underwriting) enter into an
underwriting agreement in customary form with the representative of the
underwriter or underwriters of recognized national standing, selected for such
underwriting by a majority in interest of the Requesting Holders and reasonably
acceptable to Ecogen.  Notwithstanding any other provision of this Section 7.1,
if the representative advises the Requesting Holders in writing that marketing
factors require a limitation on the number of shares to be underwritten, then
the Requesting Holders shall so advise all Holders, and the number of shares of
Registrable Securities that may be included in the registration and
underwriting shall be allocated first among all Holders thereof in proportion,
as nearly as practicable, to the respective amounts of Registrable Securities
held by such Holders at the time of filing the Registration Statement.  No
Registrable Securities excluded from the underwriting by reason of the
underwriter's marketing limitation shall be included in such registration.

                 If any Holder of Registrable Securities disapproves of the
terms of the underwriting, then such person may elect to withdraw therefrom by
written notice to Ecogen, the underwriter and the Requesting Holders.  The
Registrable Securities and/or other securities so withdrawn shall also be
withdrawn from registration; provided, however, that, if, by the withdrawal of
such Registrable Securities, a greater number of Registrable Securities held by
other Holders may be included in such registration (up to the maximum of any
limitation imposed by the underwriters), then Ecogen shall offer to all Holders
who have included Registrable Securities in the registration the right to
include additional Registrable Securities in the same proportion used to
determine the underwriter limitation in this Section 7.1(c).

                 If the underwriter has not limited the number of Registrable
Securities to be underwritten, then Ecogen and its executive officers, and such
other Persons as are determined by the Board of Directors, their successors,
and their assigns ("Other Selling





                                       31
<PAGE>   33
Stockholders"), may include securities for their own account in such
registration if the underwriter so agrees and if the number of Registrable
Securities held by the Holders that would otherwise have been included in such
registration and underwriting will not thereby be limited for any reason,
including but not limited to the price for which the Registrable Securities
will be sold.  To the extent that the underwriter wishes to limit the number of
shares to be included in the registration on behalf of Ecogen and the Other
Selling Stockholders, the shares of common stock to be registered held by the
Other Selling Stockholders shall be excluded from such offering prior to
excluding any shares held by Ecogen and those held by Ecogen shall be excluded
prior to excluding any Registrable Securities held by the Holders.

         7.2     Ecogen Registration.

                 (a)      Notice and Inclusion.  Subject to Section 6.1, if, at
any time after the Closing until the tenth anniversary of the Closing, Ecogen
shall determine to register any of its securities for its own account, other
than a registration relating solely to employee benefit plans, or a
registration relating solely to a Commission Rule 145 transaction, Ecogen
shall:

                          (i)     promptly give to each Holder written notice
         thereof (which shall include a list of the jurisdictions in which
         Ecogen intends to attempt to qualify such securities under the
         applicable blue sky or other state securities laws); and

                          (ii)    include in such registration (and any related
         qualification under blue sky laws or other compliance), and in any
         underwriting involved therein, all Registrable Securities specified
         in a written request or requests, within twenty (20) days after
         receipt of the written notice from Ecogen, by any Holder or Holders.

                 (b)      Underwriting.  If the registration of which Ecogen
gives notice is for a registered public offering by Ecogen of its securities
through an underwriting, then





                                       32
<PAGE>   34
Ecogen shall so advise the Holders as a part of the written notice given
pursuant to Section 7.2(a)(i) hereof.  In such event, the right of any Holder
to registration pursuant to this Section 7.2 shall be conditioned upon such
Holder's participation in such underwriting and the inclusion of such Holder's
Registrable Securities in the underwriting to the extent provided herein.  All
Holders proposing to distribute their securities through such underwriting
shall (together with Ecogen, and all the Other Selling Stockholders
distributing their securities through such underwriting) enter into an
underwriting agreement in customary form with the underwriter or underwriters
selected for underwriting by Ecogen.  Notwithstanding any other provision of
this Section 7.2, if the underwriter determines that marketing factors require
a limitation on the number of shares to be underwritten, then the underwriter
may exclude from such registration and underwriting some or all of the
Registrable Securities held by the Holders or the stock held by Other Selling
Stockholders in accordance with this Section 7.2(b).  Ecogen shall so advise
all Holders and all Other Selling Stockholders distributing their securities
through such underwriting, and (i) as to the first registration in which
Holders are entitled to participate pursuant to this Section 7.2, the number of
Registrable Securities and other securities that may be included in the
registration and underwriting shall be allocated among all Holders thereof on
the basis that shares held by all the Other Selling Stockholders who are not
Holders shall first be excluded to the extent required and, if further
exclusion is necessary, shares held by the selling Holders shall then be
excluded; provided, however, that, as among the respective Other Selling
Stockholders as a group on the one hand and the Holders as a group on the other
hand suffering such exclusion, the exclusion shall be in proportion, as nearly
as practicable, to the amount of securities entitled to inclusion in such
registration held by each of the Other Selling Stockholders as a group and each
of the Holders at the time of filing the Registration Statement; and (ii) as to
all subsequent registrations, the number of shares of Registrable Securities
and other securities that may be included in the registration and underwriting
shall be allocated among all Other Selling Stockholders and the Holders in
proportion, as nearly as practicable, to the respective amounts of securities
entitled to inclusion in such registration held by all such Other Selling
Stockholders and Holders at the time of filing the Registration Statement.  For
purposes of the apportionment provisions in clause (i) above, for any selling
Holder that is a partnership or corporation, the partners, retired partners,
and shareholders of





                                       33
<PAGE>   35
such Holder, the estate and family members of such partners and retired
partners, and any trusts for the benefit of any of the foregoing persons shall
be deemed to be a single "selling Holder," and any pro rata reduction with
respect to such selling Holder shall be based upon the aggregate number of
shares carrying registration rights owned by all entities and individuals
included in such "selling Holder," as defined in this sentence.  If any Other
Selling Stockholder or Holder disapproves of the terms of any such
underwriting, he may elect to withdraw therefrom by written notice to Ecogen
and the underwriter.  Any securities excluded or withdrawn from such
underwriting shall be withdrawn from such registration.

         7.3     Expenses of Registration.  All Registration Expenses incurred
in connection with any registration, qualification or compliance pursuant to
this Article VII shall be borne by Ecogen; provided, however, that the
Registration Expenses for the third and all subsequent registrations under
Section 7.1(a) hereof requested by the Holders shall be borne by the requesting
Holders pro rata on the basis of the number of their shares so registered.  All
Selling Expenses relating to the securities registered by Holders and, if
applicable, Other Selling Stockholders, and fees and disbursements of counsel,
shall be borne by the Holders or the Other Selling Stockholders, as the case
may be, of such securities pro rata on the basis of the number of their shares
so registered.

         7.4     Registration Procedures.

                 (a)      Ecogen shall use its best efforts to register or
qualify the Registrable Securities covered by a Registration Statement under
such other securities or blue sky laws of such United States jurisdictions as
Holder shall reasonably request and do any and all acts and things which may be
necessary or desirable to enable Holder to consummate the public sale or other
disposition in such jurisdictions; provided, however, that Ecogen shall not be
required in connection therewith or as a condition thereto to qualify to do
business or file a general consent to service of process in any such
jurisdictions.

                 (b)      Ecogen represents and warrants that, on the date of
its effectiveness, the Registration Statement will comply in all material
respects with the





                                       34
<PAGE>   36
applicable requirements of the Securities Act and the rules thereunder,
including without limitation Rule 415; on the date of its effectiveness, the
Registration Statement will not contain any untrue statement of a material fact
or omit to state any material fact required to be stated therein or necessary
in order to make the statements made therein not misleading; provided, however,
that no representation is made by Ecogen with respect to information relative
to any Holder; and the Prospectus will not include any untrue statement of a
material fact or omit to state a material fact necessary in order to make the
statements therein, in light of the circumstances under which they were made,
not misleading; provided, however, that no representation is made by Ecogen
with respect to information relative to any Holder.

                 (c)      If, at any time or times while the Registration
Statement is effective, Ecogen notifies Holder that a development has occurred
or is pending which, based upon consultation with Ecogen's legal counsel,
Ecogen reasonably believes may cause the then current Prospectus not to be in
compliance with applicable securities laws, then Holder shall refrain from
delivering the Prospectus and from making any offers or sales of Registrable
Securities requiring the delivery of the Prospectus until such time as Ecogen
either notifies Holder that the Prospectus complies with such laws or delivers
an amended Prospectus in replacement of the deficient Prospectus.  Ecogen shall
use its reasonable best efforts to minimize the time during which Holder must
so refrain, and no more than one (1) such period of refrain shall be imposed
during any period of one hundred eighty (180) days.

                 (d)      At least two (2) Business Days prior to the
initial filing of the Registration Statement or Prospectus and no fewer than
two (2) Business Days prior to the filing of any amendment or supplement
thereto (including any document that would be incorporated or deemed to be
incorporated therein by reference), Ecogen shall furnish each selling Holder,
its legal counsel and the managing underwriter, if any, copies of all such
documents proposed to be filed, which documents (other than those incorporated
or deemed to be incorporated by reference) shall be subject to review of such
Holder, its legal counsel and such underwriters, if any, and Ecogen shall cause
its officers and directors and the independent certified public accountants to
Ecogen to respond to such inquiries as shall be necessary, in the opinion of
respective counsel to Ecogen and any such underwriters, to





                                       35
<PAGE>   37
conduct a reasonable investigation within the meaning of the Securities Act.
Ecogen shall not file any such Registration Statement or Prospectus or any
amendments or supplements thereto to which a selling Holder, its legal counsel,
or the managing underwriters, if any, shall reasonably object on a timely basis
(i.e., within two (2) Business Days of receipt thereof).

                 (e)      Ecogen shall promptly notify each selling Holder when
the Registration Statement is declared effective; notify Holder of any
stop-order or similar proceeding by the Commission or any state securities
authority; and furnish such number of Prospectuses, Prospectus supplements and
other documents incident thereto as Holder from time to time may reasonably
request.

                 (f)      In the event of any breach by Ecogen of the
provisions of Section 7.1, 7.2, 7.3 or 7.4, the parties agree that each Holder
will suffer irreparable harm.  Accordingly, the parties agree that the
provisions of Sections 7.1, 7.2, 7.3 and 7.4 are specifically enforceable by
each Holder and that each Holder shall be entitled to temporary and permanent
injunctive relief against Ecogen and the other rights and remedies to which
each Holder may be entitled to at law, in equity or under this Agreement for
any such breach.

         7.5     Indemnification.

                 (a)      Indemnification by Ecogen.  Ecogen shall indemnify
         each Holder with respect to which registration, qualification or
         compliance has been effected pursuant to this Article VII, each of its
         officers, directors, employees, agents and partners, each Person
         controlling such Holder within the meaning of Section 15 of the
         Securities Act, each underwriter, if any, and each Person who controls
         any underwriter within the meaning of Section 15 of the Securities
         Act, against all expenses, claims, losses, damages and liabilities (or
         actions in respect thereof), including any of the foregoing incurred
         in settlement of any litigation, commenced or threatened, arising out
         of or based on any untrue statement (or alleged untrue





                                       36
<PAGE>   38
         statement) of a material fact contained in any such Registration
         Statement, Prospectus, offering circular or other document or any
         amendment or supplement thereto or incident to any such registration,
         qualification or compliance, or based on any omission (or alleged
         omission) to state therein a material fact required to be stated
         therein or necessary to make the statements therein not misleading, or
         any violation by Ecogen of the Securities Act or any rule or
         regulation thereunder applicable to Ecogen and relating to action or
         inaction required of Ecogen in connection with any such registration,
         qualification or compliance.  Ecogen shall reimburse each such Holder,
         each of its officers, directors, employees, agents and partners, and
         each Person controlling such Holder, each such underwriter and each
         Person who controls any such underwriter for any legal and any other
         expenses reasonably incurred in connection with investigating,
         preparing or defending any such expense, claim, loss, damage,
         liability or action; provided, however, that Ecogen shall not be
         liable in any such case to the extent that any such claim, loss,
         damage, liability, action or expense arises out of or is based on any
         untrue statement or omission or alleged untrue statement or omission
         made in reliance upon and in conformity with written information
         furnished to Ecogen by an instrument duly executed by such Holder or
         underwriter and stated to be specifically for use therein.

                 (b)      Indemnification by the Holders.  To the extent set
forth in the second sentence of this Section 7.5(b), each Holder shall, if
Registrable Securities or other securities held by such Holder are included in
the securities as to which such registration, qualification or compliance is
being effected, indemnify Ecogen, each of its directors, officers, employees
and agents, each underwriter, if any, of Registrable Securities covered by such
a Registration Statement, each Person who controls Ecogen or such underwriter
within the meaning of Section 15 of the Securities Act, each other such Holder,
each of such other Holder's officers, directors, employees, agents and
partners, and each Person controlling such Holder within the meaning of Section
15 of the Securities Act against all expenses, claims, losses, damages and
liabilities (or actions in respect thereof), including any of the foregoing
incurred in settlement of any litigation, commenced or threatened, arising out
of or based on any untrue statement (or alleged untrue statement) of a material
fact made





                                       37
<PAGE>   39
by the Holder and contained in any such Registration Statement, Prospectus,
offering circular or other document, or any amendment or supplement thereto or
incident to any such registration, qualification or compliance or based on any
omission (or alleged omission) to state therein a material fact required to be
made by the Holder and stated therein or necessary to make the statements
therein not misleading or any violation by Ecogen of any rule or regulation
promulgated under the Securities Act applicable to Ecogen in connection with
such registration, qualification or compliance as a result of any statement (or
based on any omission to state or alleged omission) required to be made by such
Holder.  Each such Holder shall reimburse Ecogen, such other Holders,
directors, officers, employees, agents, partners, Persons, underwriters and
control Persons for any legal or any other expenses reasonably incurred in
connection with investigating, preparing or defending any such expense, claim,
loss, damage, liability or action, in each case to the extent, but only to the
extent, that such untrue statement (or alleged untrue statement) or omission
(or alleged omission) is made in such Registration Statement, Prospectus,
offering circular or other document or any amendment or supplement thereto in
reliance upon and in conformity with written information furnished by the
Holder to Ecogen by an instrument duly executed by such Holder and stated to be
specifically for use therein; provided, however, that the obligations of such
Holders hereunder shall be limited to an amount equal to the proceeds to each
such Holder of Registrable Securities sold as contemplated herein inconnection
with the particular registration, qualification or compliance involved.

                 (c)      Notice.  Each party entitled to indemnification under
this Section 7.5 (the "Indemnified Party") shall give notice to the party
required to provide indemnification (the "Indemnifying Party") promptly after
such Indemnified Party has actual knowledge of any claim as to which indemnity
may be sought and shall permit the Indemnifying Party to assume the defense of
any such claim or any litigation resulting therefrom; provided, however, that
counsel for the Indemnifying Party, who shall conduct the defense of such claim
or any litigation resulting therefrom, shall be approved by the Indemnified
Party (whose approval shall not unreasonably be withheld), and that the
Indemnified Party may participate in such defense at its own expense; and
provided further that the failure of any Indemnified Party to give notice as
provided herein shall not relieve





                                       38
<PAGE>   40
the Indemnifying Party of its obligations under this Section 7.5 unless such
failure is materially detrimental to the Indemnifying Party.  No Indemnifying
Party, in the defense of any such claim or litigation, shall, except with the
consent of each Indemnified Party, consent to entry of any judgment or enter
into any settlement which does not include as an unconditional term thereof the
giving by the claimant or plaintiff to such Indemnified Party of a release from
all liability in respect to such claim or litigation.

         7.6     Information by Holder.  Each Holder or Holders of Registrable
Securities in any registration shall furnish to Ecogen such information
regarding such Holder or Holders and the distribution proposed by such Holder
or Holders as Ecogen may reasonably request in writing but only to the extent
as shall be required in connection with any registration, qualification or
compliance referred to in this Article VII.

         7.7     Rule 144 Reporting.  With a view to making available the
benefits of certain rules and regulations of the Commission which may permit
the sale of the Restricted Securities or Control Securities to the public
without registration, Ecogen agrees to:

                 (a)      Use its best efforts to make and keep public
information available as those terms are understood and defined in Rule 144
under the Securities Act;

                 (b)      Use its best efforts to file with the Commission in a
timely manner all reports and other documents required of Ecogen under the
Securities Act and the Exchange Act (at any time after it has become subject to
such reporting requirements);

                 (c)      For so long as a Holder owns any Restricted
Securities or Control Securities, furnish to the Holder forthwith upon request
(i) a written statement by Ecogen as to its compliance with the reporting
requirements of Rule 144 and of the Securities Act and the Exchange Act, (ii) a
copy of the most recent annual or quarterly report of Ecogen, and (iii) such
other reports and documents so filed as such Holder may reasonably request in
availing itself of any rule or regulation of the Commission allowing a Holder
to sell any such securities without registration; and





                                       39
<PAGE>   41
                 (d)      When any Holder qualifies under Rule 144 for the
unrestricted right of sale under Rule 144, Ecogen shall, upon written request
of such Holder (such request to include sufficient detail as to establish how
the Holder so qualifies under Rule 144) and an opinion of counsel satisfactory
to Ecogen, promptly remove any restrictive legend that may have been placed on
any Restricted or Control Securities and issue Ecogen Securities free of such
restrictive or other legends.

         7.8     Transfer of Registration Rights.  Monsanto may transfer the
rights to cause Ecogen to register the Registrable Securities under Sections
7.1 and 7.2 hereof to an aggregate of no more than five Holders and each such
Holder may transfer or assign such rights to a transferee or assignee in
connection with the transfer or assignment of not less than all of the shares
of the Registrable Securities held by such Holder; provided, however, that
Ecogen shall be entitled to notice of any such transfer of registration rights
within thirty (30) days of the date such transfer is effected.

         7.9     Limitations on Subsequent Registration Rights.  No owner or
prospective owner of Ecogen Securities or other securities of Ecogen shall have
any registration rights that are inconsistent with the provisions of this
Agreement.  Ecogen shall not, without the prior written consent of the Holders
(which consent shall not be unreasonably withheld) of not less than sixty-six
and two-thirds percent (66-2/3%) of the Registrable Securities then held by
Holders, enter into any agreement with any owner or prospective owner of any
Ecogen Securities or other securities of Ecogen that would allow such owner or
prospective owner to include such securities in any registration filed under
this Article VII if such inclusion would adversely affect the rights of any
Holder.

         7.10    "Market Stand-off" Agreement.  Each Holder hereby agrees that,
to the extent requested by Ecogen and an underwriter of a sale of Ecogen
Securities or other securities of Ecogen for the account of Ecogen and not for
the account of a security holder or holders exercising their respective demand
registration rights, it shall not sell or otherwise transfer or dispose of
(other than to transferees who agree to be similarly bound) any Registrable
Securities during the ninety (90) day period following the effective date of a





                                       40
<PAGE>   42
registration statement of Ecogen filed under the Securities Act; provided,
however, that all officers and directors of Ecogen, all Other Selling
Stockholders and all other Persons with registration rights (whether or not
pursuant to this Agreement) shall enter into similar agreements.  To enforce
the foregoing covenant, Ecogen may impose stop-transfer instructions with
respect to the Registrable Securities of each Holder (and the shares or
securities of every other Person subject to the foregoing restriction) until
the end of such ninety (90) day period.

         7.11    Termination of Registration Rights.  The registration rights
granted pursuant to Section 7.1 shall terminate as to each Holder on the third
anniversary of the termination or expiration of the term of the Research and
Development Agreement.

                                 ARTICLE VIII.

                          REPRESENTATIONS OF MONSANTO

         To induce Ecogen to sell the Purchased Shares described in Article II,
Monsanto represents and warrants to Ecogen as follows:

         8.1     Investment Purpose.  Monsanto is acquiring the Purchased
Shares issued pursuant to this Agreement for purposes of investment and for its
own account and not with a view to the sale or other distribution thereof as
such terms are defined in the Securities Act, as amended, and the regulations
of the Commission promulgated thereunder.

         8.2     Accredited Investor.  Monsanto is an "accredited investor" as
defined in Rule 501(a) promulgated under the Securities Act.

         8.3     Access to Information.  Monsanto has been given access to all
information requested by it regarding Ecogen, including information requested
regarding the current financial condition of Ecogen and the risks associated
therewith.





                                       41
<PAGE>   43
         8.4     Due Diligence.  Monsanto has made its own due diligence
investigation of Ecogen and Ecogen's business, and its own analysis of the
merits and risks of the investment made pursuant to this Agreement, and is not
relying on anyone else's analysis or investigation or Ecogen, its business or
the merits and risks of the purchase of the Purchased Shares other than
professionals employed specifically by Monsanto to assist Monsanto.

         8.5     Restricted Securities.  Monsanto understands that the sale of
the Purchased Shares hereunder has not been registered under the Securities Act
and that the Purchased Shares cannot be resold or transferred by Monsanto
unless it is subsequently registered under the Securities Act or unless an
exemption from such registration is available.  In this connection, Monsanto
represents that it is familiar with Rule 144 under the Securities Act as
presently in effect, and understands the resale limitations imposed thereby and
by the Securities Act.

         8.6     Exemption Reliance.  Monsanto has been advised that the
Purchased Shares are not being registered under the Securities Act or under
state securities laws, but are being offered and sold pursuant to exemptions
from such laws, and that Ecogen's reliance upon such exemptions is predicated
in part on Monsanto's representations contained herein.

         8.7     Authorization and Binding Effect.  The execution and delivery
of this Agreement and the performance by Monsanto of its obligations hereunder
are within Monsanto's corporate power, have been duly authorized by proper
corporate action on the part of Monsanto, are not in violation of, or default
under any existing law, rule or regulation of any governmental agency or
authority, any order or decision of any court, the Certificate of Incorporation
or By-Laws of Monsanto or the terms of any agreement, restriction or
undertaking to which Monsanto is a party or by which it is bound, and except
for the notification required by the Hart-Scott-Rodino Antitrust Improvements
Act of 1976, as amended, and the approval of Ecogen's shareholders of the
proposed reverse stock split as described in the Special Proxy Statement, do
not require the approval or consent of the shareholders of Monsanto, any
governmental body, agency or authority or any other person or entity.





                                       42
<PAGE>   44
         8.8     Brokers.  This Agreement was not induced or procured through
any person, firm or corporation acting as a broker or finder for Monsanto.
Monsanto agrees to hold Ecogen harmless from any loss, damage, cost or expense
resulting from any claim by any person, firm or corporation based upon any such
person, firm or corporation having acted as a broker or finder for Monsanto or
any other Person in connection with the transactions contemplated by this
Agreement, the Assignment Agreement and/or the Research and Development
Agreement.

         8.9     Stock Legend.  Subject to Section 7.7(d) hereof, certificates
representing the Purchased Shares shall bear the following legend:

         "The securities represented by this certificate have not been
         registered under the Securities Act of 1933, as amended, or under any
         state securities law and may not be sold or offered for sale in the
         absence of an effective registration statement under such act and
         applicable state securities laws or an opinion of counsel satisfactory
         to the Company that such registration is not required. The securities
         represented by this certificate are further subject to certain resale
         restrictions and entitled to the benefits set forth in an Investment
         Agreement dated as of January 24, 1996 between Ecogen, Inc., a
         Delaware corporation, and Monsanto Company, a Delaware corporation
         (the "Agreement").  A copy of the Agreement and all amendments
         thereto is on file in the office of the Secretary of the Company."

                                  ARTICLE IX.

                                 MISCELLANEOUS

         9.1     Survival of Representations and Warranties. Ecogen's
representations and warranties contained in Article III hereof and Monsanto's
representations and warranties set forth in Article VIII hereof shall survive
the Closing for a period of two (2) years.  Notwithstanding the foregoing, the
representations and warranties set forth in Sections 3.2, 3.3, 3.7 hereof, to
the extent applicable to the shares of Common Stock to be issued to Monsanto
hereunder, shall survive the Closing indefinitely.





                                       43
<PAGE>   45
         9.2     Assignment.  Except as set forth in Section 7.8, this
Agreement shall not be assigned by either party without the written consent of
the other party.  Notwithstanding the foregoing, either party may assign this
Agreement, without such consent, to the purchaser of all its assets, or of all
the assets of its business to which this Agreement or the Research and
Development Agreement relates, and Monsanto, in addition to its rights under
Section 7.8, may assign this Agreement or its rights hereunder to any
Affiliate.

         9.3     Notice.  All notices, communications and demands required or
permitted to be given or made hereunder or pursuant hereto shall be in writing
and shall be effective when delivered in person or transmitted by telegram or
telecopier (confirmed by mail), addressed as follows:

         If to Monsanto:

                          Monsanto Company
                          800 North Lindbergh Blvd.
                          St. Louis, Missouri 63167
                          Attention:  President, Ceregen
                          Telecopier Number:  314-694-7771

                          with a copy to

                          Monsanto Company
                          800 North Lindbergh Blvd.
                          St. Louis, Missouri 63167
                          Attention:  General Counsel
                          Telecopy Number:  314-694-2594

         If to any other Holder, at such address and telecopy number as such
         Holder shall have furnished Ecogen in writing.





                                       44
<PAGE>   46
         If to Ecogen:

                          Ecogen Inc.
                          2005 Cabot Boulevard West
                          Langhorne, Pennsylvania  19047
                          Attention:  President
                          Telecopy Number:  215-757-4156


                          with a copy to:

                          Dechert, Price & Rhoads
                          Princeton Park Corporate Center
                          997 Lenox Drive
                          Lawrenceville, New Jersey 08648
                          Attention: James J. Marino, Esq.
                          Telecopy Number:  609-520-3259


         Either party may change the address designated by notice given by such
party.  The parties agree to acknowledge in writing the receipt of any such
notice delivered in person.

         9.4     Governing Law.  This Agreement is deemed to have been entered
into in the State of Missouri, and its interpretation, construction, and the
remedies for its enforcement or breach are to be applied pursuant to and in
accordance with the laws of the State of Missouri (without regarding to the
conflict of laws principles thereof).

         9.5     Validity of Agreement.  If any provision of this Agreement is,
becomes, or is deemed invalid or unenforceable in any jurisdiction, such
provision shall be deemed amended to conform to applicable law so as to be
valid, legal and enforceable in such jurisdiction so deeming.  The validity,
legality and enforceability of such provision shall not in any way be affected
or impaired thereby in any other jurisdiction. If such provision cannot be so
amended without materially altering the intention of the parties, it shall be
stricken in the jurisdiction so deeming, and the remainder of this Agreement
shall remain in full force and effect.

         9.6     Waiver.  No waiver of any right under this Agreement shall be
deemed effective unless contained in a writing signed by the party charged with
such waiver, and no





                                       45
<PAGE>   47
waiver of any right arising from any breach or failure to perform shall be
deemed to be a waiver of any future such right or of any other right arising
under this Agreement.

         9.7     Entire Agreement.  This Agreement sets forth and constitutes
the entire agreement between the parties hereto with respect to the subject
matter hereof, and supersedes any and all prior agreements, understandings,
promises and representations made by either party to the other concerning the
subject matter hereof and the terms applicable hereto.

         9.8     Headings and References; Incorporation of Schedules.  The
headings contained in this Agreement are inserted for convenience of reference
only and shall not be a part, control or affect the meaning hereof.  All
references herein to Articles and Sections are to the Articles and Sections of
this Agreement.  All references herein to Schedules are to the Schedules
hereto, each of which shall be incorporated into and deemed a part of this
Agreement.

         9.9     Counterparts.  This Agreement may be executed in counterparts,
each of which shall be deemed to be an original, but which together shall
constitute one and the same instrument.

         9.10    No Presumption Against Drafter, Qualifications on Schedules.
The parties to this Agreement have jointly participated in the negotiation and
drafting of this Agreement, except for the Schedules.  In the event an
ambiguity or question of intent or interpretation arises, this Agreement,
except for the Schedules, shall be construed as if drafted jointly by the
parties and no presumptions or burdens of proof shall arise favoring any party
by virtue of the authorship of any of the provisions of this Agreement.  The
Schedules have been prepared by Ecogen and are solely the responsibility of
Ecogen even if Monsanto provided drafts of or information appearing on such
Schedules.  Any Schedule which is qualified by "knowledge", "materiality", or
words of similar import which apply to any representation or warranty which is
not similarly qualified shall be interpreted as if no qualification were stated
on the Schedule.





                                       46
<PAGE>   48
         IN WITNESS WHEREOF, the parties hereunto have executed this Agreement
as to be effective as of the date first written above.

MONSANTO COMPANY                       ECOGEN INC.


By:    Derek K. Rapp /s/               By: James P. Reilly, Jr. /s/
       ------------------                  ------------------------
Name:  Derek K. Rapp                       James P. Reilly, Jr.
Title: Director, Commercial                Chairman and Chief Executive Officer
       Partnerships & Alliances





                                       47