EXHIBIT 5.1

                                                               February 16, 2000

LJL Biosystems, Inc.
404 Tasman Drive
Sunnyvale, CA 94089

    REGISTRATION STATEMENT ON FORM S-3 (FILE NO. 333-      )

Ladies and Gentlemen:

    We have examined the Registration Statement on Form S-3 to be filed by you
with the Securities and Exchange Commission on or about February 16, 2000 (the
"Registration Statement") in connection with the registration under the
Securities Act of 1933 of shares of your common stock (the "Shares"), to be sold
by certain shareholders listed in the Registration Statement (the "Selling
Shareholders"). As your legal counsel in connection with this transaction, we
have examined the proceedings taken and are familiar with the proceedings
proposed to be taken by you in connection with the sale of the Shares.

    It is our opinion that the Shares, when sold by the selling shareholders in
the manner described in the Registration Statement, will be legally and validly
issued, fully paid and nonassessable.

    We consent to the use of this opinion as an exhibit to the Registration
Statement and further consent to the use of our name wherever it appears in the
Registration Statement and in any amendment to it.

                                          Sincerely,
                                          VENTURE LAW GROUP
                                          A Professional Corporation
                                          /s/ VENTURE LAW GROUP