Viable islets of Langerhans have been isolated from the mouse, rat, guinea-pig and human pancreas using a free hand microdissection procedure. Viablity has been assessed by light microscopy of sections stained with Gomori's aldehyde fuchsin and by measuring the insulin release from islets in vitro in response to a glucose stimulus. Ten pieces of human cadaver pancreas have been studied. Islets were isolated from 6 and in 5 cases were shown to respond to a glucose stimulus in vitro. Five pieces of human pancreas removed at operation have been studied. Islets were isolated in all cases but only 2 showed a response to a glucose stimulus. Isolated animal islets have been subjected to three preservation systems and their viability following storage noted. 1. Simple cold storage in Hank's balanced salt solution at 4 degrees C. At 15 hours 100% survival was noted. This dropped to 10% at 48 hours. There were no survivors at 72 hours. 2. Subzero storage. In group I (freezing rate 1 degree C/min) histological survival was 35% and functional survival 20%. In group II (freezing rate 5 degrees C/min with a 24-hour culture period after rewarming) histological survival was approximately 87% and functional survival 75%. 3. Organ culture. Islets from the guinea-pig, rat and mouse showed minimal morphological damage when cultured for 21 days in a simple organ culture system. At 28 days histological survival was approximately 30%. We were unable to correlate histological and functional survival in this group.