Rat allogeneic heart or kidney grafts are rejected within 6 to 8 days, whereas lymphoid cells from the same donor transplanted intravenously across the MHC barrier are eliminated, in most part, within 6 h. We have found that donor specific transfusions (DST) significantly prolonged the survival time of organ allograft but accelerated destruction of i.v. transplanted lymphocytes. Partial elimination of transplanted lymphocytes was observed after third-party blood transfusion. Blocking of the MHC class I and II determinants on transplanted lymphocytes with monoclonal antibodies OX18 and OX6 did not have effect on the lymphocyte elimination kinetics. The effect of hyperacute elimination of allogeneic lymphocytes by DST-treated rats could be adoptively transferred with their sera, although the cytotoxic antibody titer against donor MHC antigens was in these sera low or hardly detectable. DST-recipient sera contained donor-specific IgG and IgM alloantibodies. It seems that these "enhancing" antibodies could block the MHC products on organ graft endothelial cells, thereby preventing attack of circulating donor-specific cytotoxic lymphocytes. At the same time they may opsonize the transplanted lymphocytes, thereby facilitating their recognition and removal in the lymphoid organs of graft recipient.