A leptospiral genus-specific protein antigen (GP-Ag) was partially purified from Leptospira interrogans serovar kremastos strain Kyoto and canicola strain Hond Utrecht IV, by treating the organisms with Triton X-100 followed by a purification procedure including fractionation with DEAE-cellulose column chromatography and ethanol precipitation. Genus-specificity of the antigens from kremastos Kyoto and canicola was shown by the immunodiffusion (ID) test. Complement fixation (CF) test and enzyme-linked immunosorbent assay (ELISA) using GP-Ag from kremastos Kyoto showed the genus specificity of the GP-Ag. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of the antigens from kremastos Kyoto and canicola showed a major band with Coomassie blue stain. The band of GP-Ag from kremastos Kyoto was also shown by immunoprecipitation using the monoclonal antibody (GP-7) to GP-Ag from kremastos Kyoto. The molecular weight of the genus-specific antigen was estimated to be approximately 62 000. The genus specificity of the antigen was reduced by heating at 100 degrees C or treatment with proteolytic enzymes, but not reduced by treatment with sodium metaperiodate.