We previously reported that donor-specific, but not third party, skin allografts were permanently accepted if mixed allogeneic (B10+BR-->B10) reconstitution and skin graft placement were performed sequentially or simultaneously in lethally conditioned (950 cGy) recipients. The purpose of the present study was to examine whether a similar outcome would occur if islets were placed coincident with the time of bone marrow infusion and to establish the minimum dose of cytoreduction sufficient to achieve chimerism and tolerance for simultaneous islet allografts. B10 (H-2b) mice were rendered diabetic using streptozotocin. After sustained hyperglycemia (> 300 mg/dl), diabetic B10 mice were irradiated (950 cGy) and reconstituted with 5 x 10(6) T cell-depleted (TCD) B10 + 15 x 10(6) TCD B10.BR bone marrow cells. Islet allografts genetically matched or disparate to the bone marrow donor were placed under the renal capsule within 24 hr following infusion of bone marrow cells. All donor-specific B10.BR mouse (H-2k) islet allografts were permanently accepted (n = 8; MST > or = 173 days), while 7 of 9 MHC-disparate third-party BALB/c mouse (H-2d) islet grafts were rejected. The other 2 allografts remained functional over 200 days posttransplantation. We recently established a nonlethal conditioning strategy to achieve multilineage mixed chimerism. We applied this model to examine whether simultaneous islet grafts matched to the donor would be permanently accepted if the donor was incompletely myeloablated. Diabetes was induced in B10 mouse recipients. Animals with hyperglycemia were conditioned with 500 cGy of TBI followed by an infusion of 15 x 10(6) untreated B10.BR bone marrow cells. A simultaneous islet allograft matched or MHC-disparate to the bone marrow donor was performed the same day. Two days following bone marrow transplantation, a single dose of cyclophosphamide (200 mg/kg) was injected via the intraperitoneal route. Islet allografts matched to the bone marrow donor were significantly prolonged (n = 9; MST > or 226 days) and showed no evidence for chronic rejection, while MHC-disparate grafts were rejected (n = 5; MST = 34 days). Animals that received islet grafts but no bone marrow also rejected their grafts with a similar time course. These data suggest that permanent donor-specific tolerance to islet allografts placed coincident with bone marrow transplantation can be achieved after lethal as well as incompletely myeloablative conditioning.