Immunocytochemical localization of progesterone receptors in endocrine cells of the human pancreas. Progesterone receptors (PgR) have been immunocytochemically localized in the nuclei of several (40% to 75%) endocrine cells of the human pancreas and in a more variable number of neoplastic cells of 7 of 18 endocrine pancreatic tumors. Conversely the exocrine epithelial cells of the pancreas did not exhibit any PgR immunoreactivity in normal as well as in different pathologic conditions, including pancreatic adenocarcinomas. Estrogen receptors were not detected in any of the pancreatic samples investigated. Double immunocytochemical experiments have documented that PgR immunoreactivity in normal Langerhans islets is a consistent feature of most (75%) glucagon-producing A cells, of approximately 5% to 20% of insulin-producing B cells, and of a variable percentage of pancreatic polypeptide (PP)-producing cells, ranging from 5% to 70%. These figures were not affected by the sex, age, or underlying disease of the patients. The reported findings corroborate previous clinical and experimental evidence indicating that sex steroid hormones may have some regulatory effects on the functional activity of the endocrine pancreas. 