Abstract： Objective To protect the transplanted islets from the host's immune system by means of immunoexclusion membranes.Methods Rat islets were isolated from Wistar rat pancreas by ductal collagenase distention, stationary digestion, and finally with the aid of dextran gradient separation. Then the islets were encapsulated in alginate-polylysine-alginate (APA) semipermeable membranes.Results In vitro studies demonstrated that encapsulated islets secreted insulin in response to glucose challenge for at least 8 weeks, which was similar to free islets. In vivo studies showed that 15 streptozotocin (STZ)-induced diabetic mice were transplanted intraperitoneally with 1000 encapsulated islets without immunosuppression. Diabetes was reversed within 3 days, and the mice remained normoglycemic for up to 160 days, with a mean xenograft survival time of 126 days. The encapsulated islets had a significantly greater effect than unencapsulated islets, which functioned for less than 8 days.Conclusions Encapsulation of pancreatic islets in semipermeable membranes can effectively prolong xenograft survival without immunosuppression in an animal model.