The serological interrelationship of a sonicated antigen (POM-S) and an alkali-extracted "fraction 4" antigen (POM-F4) from Leptospira interrogans serovar pomona, and an ethanol-precipitated (PAT-E) and a formolized, sonicated antigen (PAT-F) from Leptospira biflexa serovar patoc were investigated. The serological responses of rabbits immunized with these antigens were examined by the enzyme-linked immunosorbent assay (ELISA), the microscopic agglutination test, and the 2-mercaptoethanol microscopic agglutination test. Antisera from these rabbits absorbed by the homologous or heterologous antigens were examined by the ELISA. The PAT-E and PAT-F antigens were genus specific and were serologically closely related but not identical. Similarly, the POM-F4 and POM-S antigens showed some serological relatedness. By the use of absorbed antisera, the serovar pomona-derived antigens were shown to be serologically related to the PAT-F but not to the PAT-E antigen in the ELISA. It is suggested that the use of several such antigens in the ELISA may reveal differences in the kinetics of the antibody response in animals infected by different leptospiral serovars.