Islets of Langerhans from unrelated donor rhesus monkeys were transplanted into the portal vein of five rhesus monkeys which had been made diabetic by partial pancreatectomy and Streptozotocin. There was definite improvement of the diabetic state with an increase in the mean KT1/2 from 0.71 to 1.52 percent per minute. The islets were isolated by a more efficient digestion-filtration process than previously had been described. This process has allowed, for the first time, successful isolation of large masses of islets permitting islet transplants in subhuman primates.