Nonopsonic binding of gonococci to human neutrophils appears to be mediated by a family of heat-modifiable outer membrane proteins termed protein IIs (PIIs). We studied the ability of a wide variety of antigonococcal monoclonal antibodies (MAbs) to inhibit the interactions of nonpiliated PII+ gonococci with human neutrophils by measuring gonococcal adherence to neutrophils and subsequent luminol-enhanced neutrophil chemiluminescence. From one set of 95 MAbs reacting with whole gonococci, only two, 7VA2 and 7B9, inhibited the ability of gonococci to induce neutrophil chemiluminescence. 7VA2 and 7B9 both reacted only with PII. MAb 53C4, from a smaller set of anti-PII MAbs, inhibited adherence to neutrophils of PII variants that bound 53C4, but not of PII variants that did not. It also inhibited gonococcus-induced neutrophil chemiluminescence. Using a whole-cell binding assay and Western blotting (immunoblotting), we showed that MAb 53C4 bound to one PII (PII4) of strain F62 and to two PIIs (PIIb and PIId) of strain FA1090. The present studies confirm and extend the role of PII in gonococcal adherence to and stimulation of human neutrophils and show intrastrain conservation of PII epitopes. The results indicate that PII is the only outer membrane component involved in adherence of nonpiliated gonococci to human neutrophils.