Immunological properties of the cells mediating delayed-type hypersensitvity to tuberculin and genetic requirements of this reaction have been studied by the method of local adoptive transfer. Peritoneal cells from BCG-immunized mice transferred the reaction into unprimed recipients without any genetic restriction. In contrast, nonadherent peritoneal cells transferred the reaction only in H-2-compatible donor-recipient strain combinations. The use of H-2 recombinant strains has shown that delayed-type hypersensitivity to tuberculin in mice is restricted by the I-A locus. Monoclonal antibody against I-Ak beta abrogated (without complement) the transfer of the reaction to CBA recipients. The phenotype of nonadherent cells, transferring the reaction has been shown to be Thy-1+, Lyt-1+, 2-, L3T4+.