Somatostatin is a cyclic polypeptide hormone originally isolated from the hypothalamus and characterised by its ability to inhibit release of growth hormone from the pituitary gland. It exists in two forms, somatostatin-14, composed of 14 amino acids and somatostatin-28, a prohormone composed of 28 residues. In the digestive system, somatostatin has been immunolocalised in intrinsic nerves of the intestinal wall and in endocrine cells of the digestive mucosa and the pancreatic islets. The antrum, duodenum and pancreas contain almost exclusively somatostatin-14, whereas the gastric body and the rest of the intestine contain 40 to 80 per cent somatostatin-28. This antibody will label D cells of the endocrine mamalian pancreas and also cells of the hypothalamic parvicellular region. This antibody may prove useful for the identification of tumours and hyperplasias of the pancreatic islets.