Pancreatic islets from DA, Lewis, or DA X Lewis F1 rats were transplanted into the portal vein of Lewis or DA recipients made diabetic by prior treatment with Streptozotocin. The islets corrected the hyperglycaemia within 48 hr but were rejected within 5 days in all combinations. Passive enhancement of homozygous Lewis or DA islets with Lewis anti-DA or DA anti-Lewis antiserum in a single dose of 500 mul delayed rejection for several days, but where DA X Lewis F1 islets were used, rejection was delayed markedly with two of five animals in both the DA X Lewis F1 to DA and in DA microliters and 2.5 ml of enhancing serum were less effective than 500 mul in suppressing rejection. Thus, passive enhancement of allogeneic pancreatic islets was extremely effective in suppressing or delaying rejection of DA X Lewis F1 islets but was able to delay rejection of homozygous pancreatic islets by only a few days.