Islets of Langerhans were isolated from the monkey pancreata by collagenase digestion method. Freshly isolated monkey pancreatic islets were transplanted under the renal capsule of normal rats. Treated group of rats received Cyclosporine A injections and the control group of rats did not receive any drug. In Cyclosporine A treated rats the monkey islets were not destroyed. They maintained their normal structural integrity with occasional neutrophils surrounding the islets. In the untreated rats dense infiltration of neutrophils destroyed the islets in three days. On the seventh day dense infiltration of lymphocytes was seen. Granulomas composed of epitheloid cells and occasional multinucleated Langerhans type giant cells were seen on the fourteenth day.