Immunological responsiveness of two lines of inbred mice, BALb/c and CBA, was studied by immunization of the animals with concentrated tick-borne encephalitis vaccine, and the role of cell-mediated immunity in formation of resistance of the two genetic lines of mice was evaluated. Determinations of the minimal immunizing dose of the vaccine revealed differences in parameters of the immunizing effect. Studies of the effect of TBE vaccine on the status of cell-mediated immunity by lymphocyte blastogenesis test led to a conclusion that vaccination of BALb/c mice contributed to the realization of the mitogenic effect of the antigen used to much greater extent.