High-affinity IL-2 receptors are expressed by T cells activated in response to foreign histocompatibility antigens but not by normal resting T cells. To exploit this difference in IL-2R expression, anti-Tac, a murine monoclonal antibody specific for the IL-2R alpha subunit, was used to inhibit organ allograft rejection. To enhance its effector function, anti-Tac was armed by chelation with yttrium-90, a pure beta-emitting radionuclide. Animals received no immunosuppression (n = 5, group I, controls), unmodified anti-Tac (n = 5, 1 mg/kg q.o.d., group II), or 90Y-anti-Tac (n = 5, 1.6 mCi/kg divided into four doses, group III). The animals in group IV (n = 4) were treated identically to those in group III with the exception that 5 micrograms/kg/dose of granulocyte colony-stimulating factor was administered intramuscularly on the days when the yttrium-90 was given and on postoperative days 12 through 35 in order to reduce hematopoietic toxicity. Mean graft survival +/- S.E.M. for the control group was 8.2 +/- 0.5 days as compared with 13.8 +/- 2.1 days (P < 0.05) for those monkeys treated with unmodified anti-Tac. Graft survival was further prolonged in animals of group III that received 90Y-anti-Tac, with a mean graft survival of 45.0 +/- 11.8 days; however, three of the five monkeys retained viable grafts within this group but died secondary to bone marrow suppression. In comparison, the monkeys in group IV that were treated with G-CSF in conjunction with 90Y-anti-Tac had a mean graft survival of 49.2 +/- 2.9 days. In contrast to group III there were no deaths in the group (IV) receiving G-CSF. Furthermore, animals in group IV had a reduced magnitude and shortened duration of irradiation-induced neutropenia when compared with that observed in group III animals that did not receive G-CSF. Thus, treatment with 90Y-anti-Tac in conjunction with G-CSF may have potential applications in organ transplantation and the treatment of IL-2 receptor-expressing neoplastic diseases.