SUMMARY Three groups of recipient dogs were conditioned for transplantation by supralethal exposure to total body irradiation from dual 60Co sources and then given hemopoietic grafts from DL-A-matched littermates. Group 1 consisted of 17 dogs given marrow cells only and not treated with immunosuppressive therapy postgrafting. Group 2 was made up of 11 dogs given a combination of marrow and peripheral blood leukocytes and no postgrafting immunosuppression. Group 3 included 13 dogs given a combination of marrow and peripheral blood leukocytes and intermittent methotrexate (MTX) therapy for 102 days postgrafting. Nine of the 17 recipients in group 1 died between 30 and 230 days after grafting with graft-versus-host (GVH) disease. Eight recipients survived beyond 230 days without GVH disease. Seven of 11 recipients in group 2 died with GVH disease after 12–151 days and 4 survived beyond 200 days. One dog in group 3 died on day 61, and 12 survived beyond 200 days without evidence of GVH disease. Thus, fatal GVH disease was seen in a number of canine recipients not given postgrafting immunosuppression despite DL-A matching with their littermate donor. The addition of peripheral blood leukocytes to the marrow inoculum did not significantly change the overall survival but increased the severity of GVH disease. Therapy with intermittent MTX postgrafting significantly prolonged survival (P < 0.01) and resulted in the establishment of stable graft-host “tolerance” in most DL-A-matched recipients.