It has been previously shown that long surviving, enhanced (AS X AUG)F1 rat kidneys residing in a primary AS recipient are not acutely rejected if transferred into a second AS recipient. The reduced immunogenicity of the retransplanted graft was attributed to a depletion of incompatible passenger cells. It is shown here that if the primary AS recipient is made chimeric by x irradiation and injection of (AS X AUG)F1 bone marrow cells, transfer of the long surviving, enhanced graft into a second AS recipient provokes acute graft rejection comparable to that observed when normal (AS X AUG)F1 kidneys are transplanted into untreated AS recipients. Transplantation of passenger cell-depleted AUG kidneys into AS recipients leads to graft rejection, with a median survival time of 22 d. Treatment of these recipients with as little as 1.5 mg/kg cyclophosphamide for 14 d induces prolonged graft survival. By contrast, five times as much cyclophosphamide treatment is required to induce prolonged survival of normal AUG kidneys (i.e., containing incompatible passenger cells) transplanted to AS recipients. These results confirm that the major alloimmunogenic stimulus of rat kidney grafts is provided by the incompatible passenger cells.