Of 261 strains of Neisseria gonorrhoeae examined for plasmids, 6 were plasmid-free, 217 contained only a small multicopy 2.6 X 10(6) dalton plasmid and 38 carried a large 24.5 X 10(6) dalton plasmid. Restriction enzyme digests and DNA-DNA hybridization studies revealed that the large plasmids isolated between 1940 and 1978 share a common core of DNA sequences (70 to 100%) and represent a group of closely related molecules.