The genetic basis of the restriction imposed on T cell mediating acquired antimicrobial resistance and delayed-type hypersensitivity (DTH) to Listeria in the rat was investigated. Sharing of MHC-coded genes between donors of sensitized T cells and antigen-stimulated recipients was both necessary and sufficient for efficient transfer of both resistance and DTH. Evidence to support this assumption was derived from experiments involving allogeneic transfers within major histocompatibility complex (MHC)-compatible strains and across MHC-barriers. Further support came from linkage studies with backcrossed rats and with the progeny of F1 rats mated with an unrelated strain. An unexpected difference in the compatibility requirements for effective transfer of DTH and resistance was noted in experiments involving the BI strain (formerly called B3). Thus, while B-region compatibility was obligatory for expression of DTH in recipients of sensitized T cells, considerable levels of protection could be transferred to either A-region or B-region compatible hosts.