Two antigenically distinct populations of Neisseria gonorrhoeae, tentatively designated Gc1 and Gc2, have been identified in 10 randomly selected gonococcal strains. The antigenic determinants responsible for this differentiation have been isolated by DEAE-cellulose chromatography from the alkali-digested phenolwater extract of these strains. Immunodiffusion, immunoelectrophoresis, and electrofocusing studies in acrylamide gel indicate that isolated antigen preparations contain essentially one component. Immunological analysis reveals that these antigens share a core determinant in addition to their respective population-specific determinants. Immunochemical and physicochemical studies demonstrate that the Gc1 and Gc2 antigens are acidic polysaccharides with very similar charge and size characteristics.