Twenty gonococcal isolates representing different serovars of the protein IA & IB serogroups, as determined by co-agglutination with monoclonal antibodies, were investigated by the restriction enzyme (RE) technique with Hind III enzyme. The patterns resulting from RE digestion and subsequent electrophoresis of the DNA fragments in gel consisted of 40-45 bands. Eight RE patterns were observed among ten of the protein IA strains representing three serovars with an identical pattern for three strains of the same serovar with positive epidemiology. Seven RE patterns were observed among ten of the protein IB strains representing five serovars with identical patterns for each of three pairs with available positive epidemiology for two of them. Seven samples from the same bacterial clone extracted and digested separately showed identical band patterns. The genetic stability was illustrated by identical patterns through 41 passages in vitro and by the same band pattern of the various virulent and avirulent colony morphology variants of a particular gonococcal strain. The results show that RE analysis of the gonococcal DNA genome is well suited as an adjunct to phenotypic markers in epidemiological studies of gonorrhoea.