The isolated islets of Langerhans are the most available donors for transplantation. As the preservation of the isolated islets is difficult, we attempted to keep these tissues viable by use of an organ culture. Islets of Langerhans from adult Wistar rats were isolated by a collagenase technique and cultured in air-CO2 (95-5%) incubator at 37 degrees C. Insulin contents of the culture media which was changed every 3 days ranged from 1097 to 1434 microunits/ml during the 80 days' culture period. Transplantation of these islets into the portal vein of streptozotocin-induced diabetic rats resulted in a good recovery from the diabetic state. These studies indicate that cultured islets do preserve their original biological abilities.