To induce mixed chimerism and renal allograft tolerance in cynomolgus monkeys, cyclophosphamide (CP) and total body irradiation (TBI) were compared as part of a nonmyeloablative conditioning regimen. CP induced dose-dependent neutropenia and lymphopenia, but hematopoietic recovery was more rapid than that observed in the TBI group. Absolute B cell counts after CP were significantly higher (P<0.01) than those in the TBI group. With CP, a total dose of 200 mg/kg with CD154 blockade regularly induced multilineage chimerism. Nevertheless, the recipients failed to achieve long-term survival because of rejection (3 of 5), posttransplantation B cell lymphoma (1 of 5), and toxicities of CP (1 of 5). As previously reported, 3 Gy of TBI with either splenectomy or CD154 blockade induced mixed chimerism and renal allograft tolerance, with significantly less morbidity and mortality than that produced by CP. Thus, TBI is more effective and less toxic than CP as part of a nonmyeloablative regimen for the induction of mixed chimerism and renal allograft tolerance in cynomolgus monkeys.