Treatment of kidney donors with procarbazine hydrochloride and methylprednisolone, respectively, 5 and 2 1/2 hr before harvesting the kidney, improved renal allograft survival in dogs significantly. Pretreatment of the donor did not have a deleterious effect on the early function of the kidney grafts. Donor blood transfused peroperatively into the recipient caused a significant reduction in survival of kidney grafts from pretreated donors, although it did not influence the survival of nontreated kidneys. Furthermore, it appeared that a peroperative injection of a suspension of nonirradiated donor lymphocytes as well as donor lymphocytes irradiated with 2,500 rad significantly decreased the survival time of pretreated kidneys. A peroperative transfusion of leukocyte-poor blood prepared with a leukocyte filtration column, which leaves erythrocytes, thrombocytes, and plasma and eliminates most of the leukocytes (99.9%), also abolished the effect of donor pretreatment. Thus, administration of donor blood constituents, whether lymphocytes or leukocyte-poor blood, can abrogate the beneficial effect of donor pretreatment on kidney graft survival. These data indicate that the effect of donor lymphocytes on the survival of pretreated kidneys is not because of a specific immunological activity of these lymphocytes but merely because of the presence of antigens on their cell surface.