The susceptibility of BALB/c, C57BL and BDF1-hybrid mouse strains to Yersinia pseudotuberculosis serovar III infection was studied. The bacterial load in the viscera and brain and the host responses at different levels, i.e. blood, peritoneal cavity and organs were determined. Blood cell parameters and peritoneal exudate cell population were evaluated during the infection using the automated hematologic analyzer Technicon H-1. It was found that BDF1-hybrid mice produced an early peritoneal inflammatory response, while in BALB/c and C57BL mice it was not observed. The high susceptibility of C57BL was associated with a great number of microorganisms in the organs and with the corresponding histological changes. It was shown that the magnitude of the inflammation induced by Y. pseudotuberculosis varied among the host strains used. The variations of the susceptibility to Y. pseudotuberculosis among inbred mouse strains suggest the possible role of genetic factors regulating the host defence.