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                                                                     EXHIBIT 5.1

                            [VENTURE LAW GROUP LOGO]



                                August 11, 1998


ISOCOR
3420 Ocean Park Boulevard
Santa Monica, CA  90405

         REGISTRATION STATEMENT ON FORM S-8

Ladies and Gentlemen:

         We have examined the Registration Statement on Form S-8 to be filed by
you with the Securities and Exchange Commission on or about August 10, 1998 (the
"Registration Statement") in connection with the registration under the
Securities Act of 1933, as amended, of a total of 350,000 shares of your Common
Stock (the "Shares") reserved for issuance under the 1992 Stock Option Plan (the
"Plan"). As your legal counsel, we have examined the proceedings taken and are
familiar with the proceedings proposed to be taken by you in connection with the
sale and issuance of the Shares under the Plan.

         It is our opinion that, when issued and sold in the manner referred to
in the Plan and pursuant to the respective agreement which accompanies each
grant under the Plan, the Shares 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 any amendments to it.

                                        Sincerely,

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


EJB