As animal-derived tissues have to be protected from immune rejection when transplanted into humans, transplants are usually accompanied by immunosuppressive therapy. However, porcine islets are preferably transplanted without the use of immunosuppressive drugs which cause significant morbidity. To protect them from immune rejection, the islets can be encapsulated in alginate microcapsules which permit the inward passage of nutrients and glucose and the outward passage of insulin. Alginate-encapsulated porcine islets transplanted without immunosuppressive drugs have survived rejection for many months in animal studies, and have been retrieved from a diabetic patient over 9.5 years after intraperitoneal transplantation and shown to contain viable islets that stain positive for insulin.