An assay of bactericidial antibody has been developed to study the host response to infection with Neisseria gonorrhoeae. This test for antibody was performed on the sera of women who were exposed to N. gonorrhoeae but who did not become infected, of patients with various types of genital infection with N. gonorrhoeae, and of a small number of individuals with no history of gonorrhea. Antibody was found in the sera of less than 31% of men and women with uncomplicated gonococcal infection. Prolonged mucosal infection with the gonococcus (greater than 33 days) correlated with the presence of bactericidal antibody. Bactericidal antibody was not detected in 95% of the specimens of acute-phase serum obtained from women with gonococcal pelvic inflammatory disease. The convalescent-phase sera of 70% of women with clinically severe pelvic inflammatory disease showed a rise in titer of bactericidal antibody to the infecting strain of N. gonorrhoeae, whereas only 11% of the convalescent-phase sera of women with mild or moderately severe disease showed a similar rise.