A cosmid gene library was prepared from Neisseria gonorrhoeae JC1, a serum-resistant clinical isolate from a patient with disseminated gonococcal infection. From this library a recombinant molecule, pWM3, was isolated which had the ability to transform F62, a serum-sensitive strain of N. gonorrhoeae, to serum resistance. This plasmid contained 2.2 kilobases of insert gonococcal DNA that coded for two peptides, one of 29 kilodaltons (kDa) and one of 17.5 kDa. Deletion of the region coding for the 29-kDa peptide resulted in the loss of the ability of the plasmid to transform F62 to serum resistance. N. gonorrhoeae F62 acquired the ability to bind blocking antibody when transformed with pWM3 or subclones that code for only the 29-kDa protein. Although similar in size, the cloned 29-kDa protein and protein III are antigenically distinct.