Ultraviolet (UV)-irradiated whole dog blood prevented the initiation of proliferative responses in allogeneic mixed lymphocyte reactions (MLR). When dogs were given 4 weekly intravenous transfusions of UV-irradiated allogeneic donor whole blood, peripheral blood lymphocytes (PBL) of the recipients responded less significantly to donor PBL in MLR for over one week after the final transfusion. Red blood cell (RBC)-crossmatching of these dogs was negative. Dogs treated with UV-irradiated blood did not produce anti-donor PBL antibody, or IgG, IgM and C3 determined by the indirect Coombs test. These dogs also had negligible delayed type hypersensitivity (DTH) responses to donor PBL at the end of the treatment period, recognized as slightly suppression of skin graft rejection. In contrast, six dogs receiving injections of untreated allogeneic whole blood evidenced high proliferation in MLR, agglutination in RBC-crossmatching, acute rejection of skin grafts and three of them produced C3 in the Coombs test.