The present studies extend our previous efforts to understand the immunological basis of specifically induced graft-versus-host (GVH) resistance in F1 hybrid rats. Immunization of F1 rats with alloreactive T-cell populations of parental strain origin induces a host-mediated T-cell response which is specific for anti-major hostocompatibility complex receptors on parental T cells. This protective immunity is rapid in onset and once induced, it provides a highly effective, specific resistance to lethal GVH disease which is radioresistant and can be adoptively transferred to syngeneic recipients.