A system of protein purification, using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electroblotting, that results in purified outer membrane proteins of the gram-negative bacterium Neisseria gonorrhoeae is described. The proteins, which ranged in apparent molecular mass from approximately 31,000 to approximately 92,000 Da, were located by naphthol blue black staining, eluted from nitrocellulose membranes using 88% formic acid, and precipitated by the addition of concentrated ammonium hydroxide. Up to 65% of the original protein present was recovered by this procedure. The resultant purified protein could then be resuspended in aqueous buffer by brief sonication, making it available for further structural and in vivo immunological analyses. Proteins purified in this manner retain their original antigenicity when probed with polyclonal and monoclonal antibodies, and are structurally unaltered by the purification process. This procedure makes it possible to acquire easily usable quantities of highly insoluble outer membrane proteins of gram-negative bacteria.