In a canine model of dog leukocyte antigen (DLA)-identical nonmyeloablative marrow transplantation including postgrafting immunosuppression with cyclosporine (CSP) and mycophenolate mofetil (MMF), engraftment was only transient with 100 cGy total body irradiation (TBI) conditioning, indicating suboptimal pretransplant immunosuppression. In contrast, grafts after 200 cGy TBI were durable in 11/12 recipients. We hypothesized that addition of pentostatin before transplantation could, in part, substitute for 100 cGy TBI. Pharmacokinetic studies showed pentostatin significantly inhibited adenosine deaminase in canine lymphocytes. Eight dogs were conditioned with 6x4 mg/m pentostatin and 100 cGy TBI, whereas two dogs received 3x4 mg/m pentostatin plus 100 cGy TBI. All were given MMF/CSP posttransplant. All showed initial engraftment; four remained stable mixed chimeras for >32 weeks. The median duration of engraftment was 13 (range 9 to >39) weeks, which was significantly longer than in six historical controls conditioned with 100 cGy TBI alone (median 10, range 3-12 weeks) (P=0.01).