In order to eliminate the adverse effects of the secretion of the exocrine tissue on the islets i n the pancreatic transplants the hosts were injected with D-L ethionine. Twenty-six male hamsters were divided into two groups. All animals received subcutaneously 1 mg per gram of body weight per day of a 5% aqueous solution of ethionine. In addition, each received 3 mg of cortisone (25 mg per ml) two days before transplant and 1 mg on the fifth and twelfth post-transplant days. In group I, the ethionine was begun seven days before transplant and was continued for six days after neonatal pancreas was placed in the cheek pouches of the hosts. In group 11, ethionine was started on the day of transplantation and was continued for 13 days. Growth and differentiation were checked for up to 56 days. Growth i n both groups was comparable to that in animals receiving cortisone alone. Histological examination revealed neither cyst formation nor interstitial secretion leakage in any of the ethionine animals. In both groups, the differentiation of islets was far superior to that found in untreated hamsters, with 85% of the hosts i n group I and 55% of those i n group I1 showing islets.