A single transfusion of 200 ml of donor blood 14 days before renal transplantation in prospectively DL-A tissue typed beagle littermates appeared to have an effect on graft survival. Seventeen per cent of the recipients did respond to the transfusion with formation of lymphocytotoxic and haemagglutinating antibodies. These "responder dogs" rejected kidney grafts in an accelerated way, compared with the "nonresponders" and with the nontreated control dogs. Responsiveness appeared to occur in pairs of littermates, which suggests that responding potency is genetically determined. There was histological evidence of acute arteritis in the renal grafts of responders, whereas cell-mediated rejection was noted in nonresponders.