Ninety two non-penicillinase producing Neisseria gonorrhoeae (non-PPNG) strains were investigated by the restriction enzyme technique with Hind III enzyme. Serogrouping by coenzyme. A with monoclonal antibodies and testing susceptibility to doxycycline were also performed by an agar dilution method. Fifteen different restriction enzyme patterns were shown, of which three represented 50% (46/92) of all strains. One band, representing a deoxyribonucleic acid (DNA) fragment of 1.45 kilobase pairs, was shared by all the strains investigated. Serogroup WI dominated, representing 57 isolates (62%) with four serovars, one of which was found in 82% (47/57) of the WI serogroups. Eight serovars were found among WII and WIII serogroups, but no one serovar dominated, as happened among WI serogroups. Identical serovars and restriction enzyme patterns were found in eight patients who were epidemiologically linked. All strains were highly or moderately sensitive to doxycycline, and the strains belonging to serogroup WI were more sensitive than those in the WII or WIII serogroups.