Immunocytochemical examination of the islets of Langerhans in various animal species, including man, indicates that insulin-producing cells (B cells), glucagon-producing cells (A cells), and cells producing somatostatin or a somatostatin-like peptide (D cells) are not randomly arranged within the islet. Whenever A cells are found in the islet—i.e., mostly in its peripheral part—they are accompanied by D cells. However, most B cells, which occupy a central position, are in contact only with other B cells. In view of the inhibitory effect of somatostatin on both insulin and glucagon secretion, it is suggested that the arrangement of A, B and D cells is important to the normal and pathological functioning of the islet.