1. Leptospires of mozdok serotype of the Pomona serogroup possessed specific pathogenic properties for a definite subspecies of Apodemus agrarius. They caused carriership in rodents of a European population, but were unable to cause such a process in Far East mice. However, leptospires of monjakov and pomona types were also unable to provoke leptospiruria in these small mammals. 2. The intensity of the formation of antibody to leptospires in different subspecies of Apodemus agrarius was dependent on the degree of their susceptibility, determined by the presence of renal leptospirosis. Leptospira-carriers among animals reacted by antibody formation 6-40 times more intensive than in those not susceptible to the infection. 3. Leptospires of pomona, monjakov and mozdok serotypes of the Pomona group taxonomically can be designated as separate subspecies, on the basis of differences in biological properties as related to their natural animal reservoirs.