A recently described method for isolation of islets of Langerhans from the human pancreas has been adapted to the cynomolgus monkey pancreas. The mean yield of islets obtained from 13 monkey pancreata was 1319 islets per gram of pancreatic tissue (range 533-1800) and the usual purity of the preparation varied from 5 to 15 per cent islet tissue. Twelve cynomolgus monkeys underwent total pancreatectomy, preparation of islets from the excised pancreas and autotransplantation to either the spleen or the liver. Three animals received no transplant and became immediately diabetic, surviving 4-8 days. Four animals became normoglycaemic after intrasplenic islet transplantation, and survived 6 weeks, at which time splenectomy was performed with immediate onset of diabetes. Splenic vein insulin sampling confirmed the spleen as the source of insulin, and histological examination showed implanted islet tissue in all cases. Five animals became normoglycaemic after intrahepatic islet implantation. Three animals subsequently became diabetic at 4 and 5 months and two animals still have functioning grafts, the longest function being 9 months. These results suggest that in the cynomolgus monkey sufficient islets can be extracted from a single donor pancreas to reverse diabetes.