Enhancement of (AS X Aug)F1 pancreatic islet allograft survival in diabetic AS rats was achieved by treating the recipient with a course of antidonor antiserum. Their survival in both untreated and treated recipients was compared with the survival of transplanted heart, kidney, and skin allografts similarly treated. The islet allografts in the untreated group were rejected by a mean of 3.2 days whereas the other allografts were rejected by a mean of 8.3 days. In the enhanced group all of the kidney allografts, 7 of 11 heart allografts, and 4 of 10 islet allografts had a prolonged survival, whereas none of the skin allografts had a prolonged survival. Islet allografts are thus more susceptible to rejection than other tissues, but their survival can be prolonged using specific immunosuppression. The reasons for the increased susceptibility to rejection are discussed.