This report concerns the relationships between patients' uncomplicated genital infections due to Neisseria gonorrhoeae and the bactericidal activity of their sera against their infecting organisms. Forty isolates of N. gonorrhoeae and 91 serum samples were obtained from 23 patients with their first or repeated gonococcal infections. Of 32 sera obtained at the time of infection, 14 (44%) were bactericidal for the infecting strains. Of convalescent-phase sera from 27 infections, 12 (44%) were bactericidal; 22%-33% of convalescent-phase sera had increased bactericidal activity when compared with acute-phase sera or sera obtained before infection. Eleven patients developed 17 new infections after sera and negative cultures were obtained; in six (35%) the previously obtained sera were bactericidal but did not prevent infection.