Antibody spectra in the sera of guinea pigs and white mice infected with Y. pestis and Y. pseudotuberculosis, the causative agents of plague and pseudotuberculosis, were determined with the use of immunoblotting. The amount of antigens, found to produce humoral immune response both in plague and in pseudotuberculosis was, on the whole, considerably less (5-6 times) in white mice than in guinea pigs. The animals were found to produce antibody response to plasmid-specific (Yop) Y. pestis antigens with with molecular weights 76, 41 and 34 kD and Y. pseudotuberculosis antigens with molecular wt of 41, 34 and 25 kD. Differences in the character of humoral immune response induced in guinea pigs by killed and live Y. pseudotuberculosis were determined. In the animals inoculated with killed bacteria these differences consisted in the absence of antibody formation to the immunodominant antigen with a molecular weight of 62 kD and in the increase of antibody response to antigens with molecular weights of 36 and 21 kD, supposed to be related to Y. pseudotuberculosis pathogenicity factors and to possess protective properties.