A role of macrophage-mediated inflammatory events in early islet graft loss is increasingly acknowledged. Osteopetrotic mice (op/op) have a complete absence of CSF-1, and thus of most tissue macrophages. We have investigated whether the absence of CSF-1-dependent macrophages in the graft itself or at the transplant site could decrease the delay to function of a syngeneic marginal islet mass. Islets transplanted into op/op or control recipients reversed diabetes in 59 days vs. 10 days (p = 0.28, NS). Islets isolated from op/op or control mice reversed diabetes in 11 days vs. 10 days. IL-1 and TNF-alpha release by cultured islets was markedly decreased for op/op islets compared with control islets (IL-1: 0 vs. 4.2 pg/ml, p = 0.07; TNF-alpha: 67 vs. 311 pg/ml, p = 0.002). In contrast, IL-6 release by op/op islets was significantly increased (11.1 vs. 4.3 ng/ml, p = 0.006). CSF-1-dependent tissue macrophages may not be critical in the inflammatory insult to islet transplants. Alternate patterns of intraislet release of deleterious proinflammatory cytokines may exist.