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                                                                     EXHIBIT 5.1


                                 August 3, 2000


(949) 451-3800


La Jolla Pharmaceutical Company
6455 Nancy Ridge Drive
San Diego, California 92121

         Re: Registration Statement on Form S-3 of
             La Jolla Pharmaceutical Company

Ladies and Gentlemen:

         We refer to the registration statement on Form S-3 (the "Registration
Statement") under the Securities Act of 1933, as amended (the "Securities Act")
filed by La Jolla Pharmaceutical Company, a Delaware corporation (the
"Corporation"), with the Securities and Exchange Commission (the "Commission")
on the date hereof in connection with the registration under the Securities Act
of 4,800,000 shares of the Corporation's common stock, par value $0.01 per share
(the "Shares") issued by the Corporation pursuant to that certain Stock Purchase
Agreement, dated as of July 19, 2000, by and between the Corporation and the
purchasers listed on Schedule A thereto (the "Stock Purchase Agreement").

         For purposes of rendering this opinion, we have examined the originals
or certified copies of such corporate records, certificates of officers of the
Corporation and/or public officials and such other documents, including the
Stock Purchase Agreement, and have made such other factual and legal
investigations, as we have deemed relevant, necessary or appropriate. In such
examination, we have assumed the genuineness of all signatures, the conformity
to original documents of all documents submitted to us as conformed or
photostatic copies and the authenticity of the originals of such copies.

         Based on our examination described above, subject to the assumptions
stated above and relying on the statements of fact contained in the documents
that we have examined, we are of the opinion that the Shares have been duly
authorized and are validly issued, fully paid and non-assessable.

         We render no opinion herein as to matters involving the laws of any
jurisdiction other than the federal laws of the United States of America and the
laws of the State of Delaware. We are not admitted to practice in the State of
Delware; however, we are generally familiar with the Delware General


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La Jolla Pharmaceutical Company
August 3, 2000
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Corporation Law as presently in effect and have made such inquiries as we
consider necessary to render the foregoing opinions. This opinion is limited to
the effect of the current state of the laws of the United States of America and,
to the limited extent set forth above, the State of Delaware and to the current
judicial interpretations thereof and to the facts bearing upon this opinion as
they currently exist. We assume no obligation to revise or supplement this
opinion in the event of future changes in such laws or interpretations thereof
or such facts.

         We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement, and we further consent to the use of our name under the
caption, "Legal Matters" in the Prospectus forming a part of said Registration
Statement. In giving this consent, we do not thereby admit that we are within
the category of persons whose consent is required under Section 7 of the
Securities Act or the Rules and Regulations of the Commission.


                                             Very truly yours,


                                             /s/ GIBSON, DUNN & CRUTCHER LLP
                                             ---------------------------------
                                                 Gibson, Dunn & Crutcher LLP