The role of gonococcal antigens in serum bactericidal activity was determined for an isolate of Neisseria gonorrhoeae from a patient with disseminated gonococcal infection (DGI). Gonococcal outer membranes were purified by differential ultracentrifugation of sheared organisms treated with EDTA. The membranes were solubilized in an endotoxin-disaggregating buffer, and the proteins were separated from the endotoxin by molecular sieve chromatography. Chemical characterization of the endotoxin from the DGI strain revealed the presence of heptose (7.8% of carbohydrate composition) and 2-keto-3-deoxyoctonate (6.1%, wt/wt) in concentrations similar to rough endotoxins of gram-negative enteric bacteria. Dermal Schwartzman reactions were positive for this endotoxin (4 μg) and the corresponding outer membrane (139 μg), but negative for the protein fraction (>500 μg). The patient's whole serum or the IgG fraction, each with complement, reduced the number of the infecting organisms by greater than 1 log₁₀ in a bactericidal assay. Outer membrane and its protein and endotoxin fractions (0.8-500 μg) from the DGI strain were separately preincubated with the patient's convalescent serum and specific inhibition of bactericidal activity occurred with the endotoxin fraction (25 μg) and the outer membrane (100 μg); the protein (500 μg) exhibited no inhibition. Similar results were obtained by inhibiting the bactericidal activity of rabbit antiserum, prepared by intravenous inoculation of an isolate from a patient with pelvic inflammatory disease, with antigen purified from that strain. That this was specific immune inhibition and not anticomplementary activity was shown by the failure of these antigens to inhibit other complement-dependent bactericidal systems.