STOCK PURCHASE ELECTION
        TO EXERCISE PURCHASE RIGHTS AND TO TENDER SHARES OF COMMON STOCK
           (INCLUDING THE ASSOCIATED PREFERRED STOCK PURCHASE RIGHTS)
                                       OF
 
                              MYCOGEN CORPORATION
           PURSUANT TO THE OFFER TO PURCHASE DATED SEPTEMBER 4, 1998
                                       OF
                         AGROSCIENCES ACQUISITION, INC.
                         A MAJORITY-OWNED SUBSIDIARY OF
                              DOW AGROSCIENCES LLC
                   AND A WHOLLY-OWNED INDIRECT SUBSIDIARY OF
                            THE DOW CHEMICAL COMPANY
 
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                          SIGNATURE REQUIRED ON PAGE 4
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Dear Purchase Rights Holder:
 
    AgroSciences Acquisition Inc. ("Purchaser"), a majority-owned subsidiary of
Dow AgroSciences LLC ("Parent") and a wholly owned indirect subsidiary of The
Dow Chemical Company ("TDCC"), has made an offer to purchase all of the
outstanding shares of common stock, par value $0.001 per share (including the
associated preferred stock purchase rights) (the "Shares"), of Mycogen
Corporation (the "Company"), at a purchase price of $28.00 per Share, net to the
seller in cash. The enclosed Offer to Purchase dated September 4, 1998 (the
"Offer to Purchase"), and the related Letter of Transmittal enclosed with this
Stock Purchase Election (which, together with the Offer to Purchase, constitute
the "Offer"), describe the Offer in greater detail.
 
    As a holder of purchase rights ("Purchase Rights") to purchase Shares under
the Mycogen Corporation 1995 Employee Stock Purchase Plan (the "Stock Purchase
Plan"), you may submit to the Company this Stock Purchase Election exercising
all of your Purchase Rights (calculated assuming your payroll deductions had
continued at the same level through November 30, 1998) and the Company will
tender each of the Shares issuable under all such Purchase Rights (the "Stock
Purchase Shares") in the Offer, as set forth below under "Instructions";
provided that any exercise of Purchase Rights must be in accordance with the
terms of the Stock Purchase Plan.
 
    By signing below, you hereby agree that, immediately prior to the purchase
of Shares by Purchaser in the Offer, and contingent upon such purchase, you will
be deemed to have fully exercised your Purchase Rights and to have tendered each
of the Stock Purchase Shares to Purchaser pursuant to the Offer. By signing
below, you also agree that the amount of payroll deductions accumulated in your
account under the Stock Purchase Plan will be used to exercise the Purchase
Rights and that the difference between the entire purchase price for the Stock
Purchase Shares and the amount of the accumulated payroll deductions shall be
deemed to be paid with the proceeds of an interest free advance from the Company
(the "Advance"). The Advance will be deemed to be repaid in full on your behalf
by Purchaser from a portion of the consideration due to you for such Stock
Purchase Shares in the Offer. After such repayment, you will be entitled to
receive from Purchaser the balance of such consideration with respect to each
Stock Purchase Share purchased by Purchaser pursuant to the Offer.
 
    By signing below, you acknowledge that you have been advised that (1) the
Company and Parent will make it possible for Stock Purchase Shares issuable upon
exercise of Purchase Rights covered by Stock Purchase Elections above to be
deemed tendered in the Offer, and (2) upon the purchase of Stock Purchase Shares
pursuant to this Stock Purchase Election, you will have no further rights under
such Purchase Rights.
 
    TO ASSURE THAT YOUR STOCK PURCHASE ELECTION CAN BE PROCESSED ON TIME, PLEASE
EXECUTE THIS STOCK PURCHASE ELECTION AND DELIVER IT TO THE COMPANY ACCORDING TO
THE INSTRUCTIONS SET FORTH BELOW, BEFORE 5:00 P.M., SAN DIEGO, CALIFORNIA TIME,
ON WEDNESDAY, SEPTEMBER 30, 1998, UNLESS THE OFFER IS EXTENDED.
 
    The Offer is being made in connection with an Agreement and Plan of Merger
(the "Merger Agreement") dated as of August 31, 1998, among the Company,
Purchaser, Parent and, for the limited purpose set forth in the Merger
Agreement, TDCC. If you decide not to execute this Stock Purchase Election and
return it to the Company and, thereby, not exercise your Purchase Rights and
tender your Stock Purchase Shares under the Offer, then, contingent upon the
purchase by Purchaser of Shares pursuant to the Offer, the Company intends to
treat your Purchase Rights as terminated and no longer outstanding as of the
Effective Time of the Merger (as defined in the Offer to Purchase) and your
payroll deductions will be returned to you; provided, however, that the Company
will make arrangements so that if you consent to the termination of your
Purchase Rights (either before the Effective Time of the Merger, or within a
reasonable time thereafter), then you will be entitled to receive in respect of
your Purchase Rights an amount in cash equal to the net amount you would have
received if you had executed the Stock Purchase Election.
 
    If you require additional information concerning the terms and conditions of
the Offer, please call Georgeson & Company Inc., the Information Agent, at (800)
223-2064. If you require additional information

concerning the procedure to tender Stock Purchase Shares or the completion of
this Stock Purchase Election, please call Cheri Manis of the Company's Investor
Relations Department at (800) 745-7475.
 
    BEFORE COMPLETING THIS FORM, PLEASE READ CAREFULLY THE ACCOMPANYING OFFER TO
PURCHASE AND ALL OTHER ENCLOSED MATERIALS.
 
    THE SPECIAL COMMITTEE COMPRISED OF TWO MEMBERS OF THE COMPANY'S BOARD OF
DIRECTORS INDEPENDENT OF TDCC, PARENT AND PURCHASER (THE "SPECIAL COMMITTEE"),
UNANIMOUSLY RECOMMENDED TO THE COMPANY'S BOARD OF DIRECTORS THAT IT APPROVE THE
OFFER. THE COMPANY'S ENTIRE BOARD OF DIRECTORS ALSO REVIEWED THE OFFER AND,
AFTER RECEIPT OF THE RECOMMENDATION OF THE SPECIAL COMMITTEE, CONCLUDED THAT THE
OFFER IS IN THE BEST INTERESTS OF THE COMPANY AND ITS STOCKHOLDERS (OTHER THAN
TDCC OR ITS AFFILIATES). ACCORDINGLY, THE COMPANY'S BOARD OF DIRECTORS
UNANIMOUSLY HAS APPROVED THE OFFER AND RECOMMENDS THAT STOCKHOLDERS OF THE
COMPANY ACCEPT THE OFFER AND TENDER THEIR SHARES.
                            ------------------------
 
                                  INSTRUCTIONS
 
    Carefully complete this Stock Purchase Election below. To assure that your
Stock Purchase Election can be processed on time, please be sure to sign and
date the form and return this Stock Purchase Election to Mycogen Corporation,
AgroSciences Tender Offer, 5501 Oberlin Drive, San Diego, California 92121,
Attention: Cheri Manis, not later than 5:00 p.m., San Diego, California time, on
Wednesday, September 30, 1998, unless the Offer is extended.
 
    The Company reserves the absolute right to waive any defect or irregularity
in the exercise of any Purchase Rights or the tender of any Stock Purchase
Shares. No exercise of Purchase Rights and tender of Stock Purchase Shares will
be deemed to be properly made until all defects or irregularities have been
cured or waived. None of the Company, the Dealer Manager, the Depositary, the
Information Agent or any other person is or will be obligated to give notice of
any defects or irregularities in tenders of exercises of Purchase Rights and
Stock Purchase Shares, and none of them will incur any liability for failure to
give any such notice.
 
    THE METHOD OF DELIVERY OF THIS DOCUMENT IS AT THE ELECTION AND RISK OF THE
TENDERING HOLDER OF PURCHASE RIGHTS. NO FACSIMILE TRANSMISSIONS OF THE STOCK
PURCHASE ELECTION WILL BE ACCEPTED. IF DELIVERY IS BY MAIL, REGISTERED MAIL WITH
RETURN RECEIPT REQUESTED IS RECOMMENDED. IN ALL CASES, SUFFICIENT TIME SHOULD BE
ALLOWED TO ASSURE DELIVERY.
 
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STOCK PURCHASE EXERCISE
 
    If you want to exercise your Purchase Rights and tender your Stock Purchase
Shares in the Offer, follow the instructions below.
 
    THE OFFER AND WITHDRAWAL RIGHTS EXPIRE AT 12:00 MIDNIGHT, NEW YORK CITY
TIME, ON FRIDAY, OCTOBER 2, 1998, UNLESS PURCHASER EXTENDS THE OFFER. EXCEPT AS
SET FORTH HEREIN, IF YOU WANT TO PARTICIPATE IN THE OFFER, YOU MUST COMPLETE AND
RETURN THE ENCLOSED STOCK PURCHASE ELECTION AS SET FORTH IN INSTRUCTION (2)
BELOW PRIOR TO THE EXPIRATION OF THE OFFER.
 
    To properly complete your Stock Purchase Election, you need to do the
following:
 
    (1) Complete, date and sign the Stock Purchase Election on page 4.
 
    (2) Return this Stock Purchase Election to Mycogen Corporation, AgroSciences
Tender Offer, 5501 Oberlin Drive, San Diego, California 92121, Attention: Cheri
Manis, not later than 5:00 p.m., San Diego, California time, on Wednesday,
September 30, 1998, unless the Offer is extended. Stock Purchase Elections
received after the expiration of the Offer will not be honored. NO FACSIMILE
TRANSMITTALS OF THE STOCK PURCHASE ELECTION WILL BE ACCEPTED.
 
WITHDRAWAL
 
    If completely and properly submitted, your direction to exercise Purchase
Rights and tender the related Stock Purchase Shares will be deemed irrevocable
upon receipt by the Company unless withdrawn prior to the expiration of the
Offer, unless the Offer is extended. In order to make an effective withdrawal,
you must submit a new Stock Purchase Election which may be obtained by calling
Cheri Manis of the Company's Investor Relations Department at (800) 745-7475 (or
use a photocopy of a Stock Purchase Election). Your new Stock Purchase Election
must be signed and dated on page 4. You must also write "WITHDRAW" in the space
beneath the signature block on page 4. Upon receipt of a new, signed, dated and
properly completed Stock Purchase Election, your previous direction will be
deemed canceled. You may be deemed to re-exercise your Purchase Rights and be
deemed to re-tender your Stock Purchase Shares by obtaining another Stock
Purchase Election from Cheri Manis (or use a photocopy of a Stock Purchase
Election) and repeating the previous instructions for directing exercises and
tenders as set forth above.
 
FURTHER INFORMATION
 
    If you require additional information concerning the terms and conditions of
the Offer, please call Georgeson & Company Inc., the Information Agent, at (800)
223-2064. If you require additional information concerning the procedure to
tender Stock Purchase Shares receivable upon exercise of your Purchase Rights or
the completion of this Stock Purchase Election, please call Cheri Manis of the
Company's Investor Relations Department at (800) 745-7475.
 
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                                   SIGNATURE
                                   (REQUIRED)
 
      The undersigned acknowledges receipt of the Offer to Purchase, dated
  September 4, 1998, from Purchaser and Parent and represents that the
  undersigned has carefully read such documents. The undersigned hereby
  instructs the Company, subject to the terms and conditions set forth in this
  Stock Purchase Election and the Offer to Purchase, to carry out the
  instructions contained in this form.
 
      The Company is hereby authorized to exercise all Purchase Rights of
  which the undersigned is a holder and to tender the undersigned's Stock
  Purchase Shares.
 
      The undersigned understands that withholding taxes, at the minimum rate
  or the rate specified in Form B tax election previously filed with the
  Company, will be withheld from any proceeds received by the undersigned
  (unless the undersigned has submitted with this form, or pursuant to
  subsequent notification from the Company, a check in an amount sufficient to
  cover such amount). The undersigned further agrees that, if such proceeds
  are insufficient to cover applicable withholding taxes, Stock Purchase
  Shares will not be credited to his or her account until he or she has, upon
  request of the Company, forwarded to the Company a check in an amount
  sufficient to cover such taxes. In lieu of withholding at such rates, the
  undersigned instructs the Company to withhold taxes at the following rates
  (which may not be less than the minimum federal, state or local tax rates
  applicable to you, nor in excess of such proceeds):
             Federal________%;  State:________%;  Local:________%.
 
      THE METHOD OF DELIVERY OF THIS DOCUMENT IS AT THE ELECTION AND RISK OF
  THE UNDERSIGNED.
 
  Signature:________________________              Date:_______________________
  Name:_______________________  Social Security Number:_______________________
                  (PLEASE PRINT)
 
  Address:____________________________________________________________________
          (STREET ADDRESS, INCLUDING APARTMENT NUMBER -- PLEASE PRINT)
 
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   (CITY)                          (STATE)                         (ZIP CODE)
 
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