Rat islets encapsulated in alginate-polylysine membranes were implanted intraperitoneally into nonimmu-nosuppressed streptozocin-induced diabetic mice. Diabetes was reversed within 3 days, and the animals remained normoglycemic for up to 144 days, with a mean xenograft survival of 80 days. This was significantly greater than nonencapsulated islets, which functioned for 80%. Xenografts of rat islets encapsulated in alginate-polyornithine membranes also had a prolonged survival rate. This study demonstrates that encapsulation of pancreatic islets in semipermeable membranes can prolong xenograft survival in the absence of immunosuppreSSion.