Serological classification of Neisseria gonorrhoeae by co-agglutination (COA) into previously described antigen classes W and J was confirmed in the present work. Immunization of rabbits with classified organisms gave antibodies which produced the expected results when used in the preparation of COA reagents. Colour/opacity colonial variants of isogenic strains, characterized by stereomicroscopy as opaque and transparent, respectively, were found to influence immunization, absorption, co-agglutination and precipitation in immunoelectrophoresis (IE). It was shown by COA and IE that organisms of opaque colonies often contained an extra antigenic factor(s) which resisted heating at 100 degrees C but was sensitive to treatment with pronase. They were extracted by heating the organisms in saline or lithium chloride solution. Serological classification with COA in clinical isolates was reproducible with reagents for antigen classes W and J. Colony morphology dependent reactions, mostly due to organisms of opaque colonies, occurred in 7% with reagents for antigen class W and in 20% for antigen class J.