Exhibit 5.1

Goodwin Procter llp
Counselors at Law
Three Embarcadero Center,
24th Floor
San Francisco, CA 94111
T: 415.733.6000
F: 415.677.9041

September 1, 2010

La Jolla Pharmaceutical Company
4365 Executive Drive, Suite 300
San Diego, CA 92121

Re: Securities Being Registered under Registration Statement on Form S-8

Ladies and Gentlemen:

We have acted as counsel to you in connection with your filing of a Registration Statement on Form S-8 (the “Registration Statement”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), on or about the date hereof relating to the registration of an aggregate of 19,400,000 shares (the “Shares”) of Common Stock, $0.0001 par value per share, of La Jolla Pharmaceutical Company, a Delaware corporation (the “Company”), that may be issued pursuant to the Company’s 2010 Equity Incentive Plan, 1995 Employee Stock Purchase Plan, as amended, and option agreements with individual officers (collectively, the “Plans”).

We have reviewed such documents and made such examination of law as we have deemed appropriate to give the opinions expressed below. We have relied, without independent verification, on certificates of public officials and, as to matters of fact material to the opinion set forth below, on certificates of officers of the Company.

The opinion expressed below is limited to the Delaware General Corporation Law (which includes reported judicial decisions interpreting the Delaware General Corporation Law).

For purposes of the opinion expressed below, we have assumed that a sufficient number of authorized but unissued shares of the Company’s Common Stock will be available for issuance when the Shares are issued.

Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, upon issuance and delivery against payment therefor in accordance with the terms of the Plans, will be validly issued, fully paid and non-assessable.

We hereby consent to the inclusion of this opinion as Exhibit 5.1 to the Registration Statement. In giving our consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.

Very truly yours,
 

/s/ Goodwin Procter LLP  

GOODWIN PROCTER llp