An in vitro bactericidal assay that utilized a set of 20 typing antisera to Neisseria meningitidis was developed to distinguish differences in strains of Neisseria gonorrhoeae based on patterns of killing. When test conditions were rigidly standardized, the method was sensitive and reproducible. Strains of gonococci from 20 unrelated patients had 20 different patterns of killing. Sets of strains of N. gonorrhoeae that were likely to be identical on the basis of clinical histories were examined. Organisms isolated from consorts reacted in an identical fashion in seven of nine instances. The two nonidentical sets of strains from consorts were also different in their antibiotic sensitivity patterns. The case history of one patient suggested that one of these strains was obtained from a source of infection other than the named consort, and a second patient was reinfected with a strain from her partner four months later. Four sets of strains were cultured at different intervals after appropriate therapy. On the basis of bactericidal patterns, the strains from one patient were the same, a finding that suggested treatment failure; the strains from other patients were not identical, a fact that suggested that patients were reinfected.