A canine model of marrow transplantation was used to further define the host cells mediating resistance to marrow engraftment. Recipient dogs were given 9.2 Gy of total body irradiation followed by marrow infusion from unrelated DLA-nonidentical donors. No postgrafting immunosuppression was given. At three and ten days posttransplantation recipient marrow and peripheral blood cells were obtained and characterized by the following in vitro studies: morphologic analysis; phenotypic analysis with monoclonal antibodies; assays for natural killer cell (NK) activity; and cocultures with donor marrow to study the effect on donor CFU-GM growth. Daily differential cell counts revealed a proliferation of peripheral blood mononuclear cells approximately eight days posttransplant. By day 10 surviving host cells were uniformly large granular lymphocytes which were phenotypically of T cell lineage, had NK activity, and were capable of suppressing donor marrow CFU-GM growth. Mononuclear cells from dogs given total body irradiation only and no marrow infusion (radiation control group), did not suppress CFU-GM growth when cocultured with marrow from unrelated DLA-mismatched dogs. These results suggest that radioresistant host cells with the morphology of large granular lymphocytes and NK activity and which proliferate in response to the infused donor marrow cells mediate resistance to DLA-nonidentical marrow grafts. It remains to be determined, however, whether in vitro functional studies reflect the mechanisms involved in vivo.