Previous investigations had demonstrated that genus-specific and species-specific antigens of leptospires are deep-seated within the leptospiral cell. Conversely, the present study has shown that strain CH11, serovar andamana of the non-pathogenic species of Leptospira biflexa is endowed on its surface with two cross-reacting antigens; a newly recognized antigen common to L. interrogans and L. biflexa spp. and an antigenic determinant common to a previously described genus-specific protein antigen (GP-Ag). The serovar andamana, when thimerosal-treated, behaved like an interspecies-specific antigen, cross-reacting in the complement fixation test with sera from rabbits immunized with L. interrogans and L. biflexa spp. and with sera from subjects with leptospirosis from various serovars. In the immuno electron microscopic test, human leptospirotic sera bound to the surface of thimerosal-treated andamana and not to the surface of untreated andamana showing that an interspecies-specific antigen was located underneath the outermost layer of this serovar. In the same test, the monoclonal antibody GP-7 against the GP-Ag bound to the surface of untreated andamana and not to the surface of thimerosal-treated andamana, showing that an antigenic determinant, common to GP-Ag and different from the first one, was located on the outer membrane of andamana. In human leptospirotic sera, antibodies against the cross-reacting antigen of thimerosal-treated andamana demonstrated by the complement fixation test were formed earlier and in a higher percentage of sera than the serovar-specific antibodies against 16 L. interrogans serovars demonstrated by the microagglutination test.