Both transplantation immunity (homograft sensitivity) and hemagglutinin response were observed in mice immunized with a skin graft or an injection of various splenic preparation in A/He to CBA strain combinations. Rejection of the first-set skin grafts, at approximately the 10th day after tansplantaion, wast always followed by hemagglutimin production in the recipient serum. Consequently, rejection of the second-set skin grafts (applied 7 days after the first rejection) by the same recipients took place, at approximately the 6th day, in the presence of hemagglutinins. A single injection of intact or disrupted cells resulted in an accelerated rejection of the challenging graft (applied 7 days after injection), yet no hemagglutinins could be detected prior to rejection. Conversely, no accelerated rejection was observed in mice immunized with stored (at 4C for 3days) disrupted spleen cell preparation which, prior to injection, was incorporated into Freund's complete adjuvant, despite the detectable hemagglutinin response at the time of graft challenge. The time difference between the duration of transplantation immunity and the appearance of hemagglutinin indicated that the latter does not seem to play a major role in graft rejection. Thus it was possible, under appropraite experimental condition, to disassociate transplanation immunity from hemagglutinin response.