Exhibit 10.31


                               FIRST AMENDMENT TO
                   PURCHASE AGREEMENT AND ESCROW INSTRUCTIONS


         THIS FIRST AMENDMENT TO PURCHASE AGREEMENT AND ESCROW INSTRUCTIONS
("Amendment") is entered into at San Diego, California as of February 12, 2002,
between H. G. FENTON COMPANY, a California corporation which acquired title as
H. G. Fenton Material Company ("Seller"), and BIOSITE INCORPORATED, a Delaware
corporation ("Buyer"), with reference to the following

                                R E C I T A L S:

         A.       Seller and Buyer are the parties to a Purchase Agreement and
Escrow Instructions dated as of December 7, 2001 ("Agreement"), relating to that
certain real property consisting of approximately 34.7 gross acres in the
Carroll Canyon area of San Diego, California ("Land"). Initially capitalized
terms not otherwise defined in this Amendment have the same meanings as in the
Agreement, as previously amended. Pursuant to the Agreement, the parties have
established Escrow No. 51943-PM with Stewart Title of California, Inc. as Escrow
Holder.

         B.       The Contingency Date under the Agreement was the later to
occur of (i) December 14, 2001, or (ii) ten (10) Business Days after Buyer's
receipt of the CC&Rs. Buyer received the CC&Rs with all exhibits thereto on
February 4, 2002, with the result that the initially established Contingency
Date became February 13, 2002. The parties now wish to extend the Contingency
Date by two days.

         THE PARTIES AGREE:

         1.       EXTENSION OF CONTINGENCY DATE. The parties agree that
Paragraph 2.1.16 of the Agreement shall be amended to read in its entirety as
follows:

                  "CONTINGENCY DATE" means February 15, 2002.

         2.       OTHER MATTERS OF AGREEMENT.

                  (a)      This Amendment may be executed in any number of
counterparts, each of which will be deemed to be an original, but all of which
together will constitute one instrument.

                  (b)      Except to the extent modified hereby, all provisions
of the Agreement shall remain in full force and effect.


SELLER:                                        BUYER:

H. G. FENTON COMPANY, a California             BIOSITE INCORPORATED, a Delaware
corporation                                    corporation

By                                             By
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Its                                            Its
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By                                             By
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Its                                            Its
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