In order to correlate the different cell types of the human endocrine pancreas to a specific secretion product, an immunoelectron microscopic localization of the hormones whose production had been attributed to pancreatic islets was conducted. Glucagon and insulin were respectively localized in the typical A- and B-cells, whereas no subclasses of A-cells could be identified. With antibodies that reacted with the gastrin cells in the human gastric mucosa, it was not possible to detect gastrin in any of the islet cell types. In confirmation of recent results obtained by light microscopy, somatostatin was found in all the typical D-cells containing large, weakly electron-dense secretory granules. The human pancreatic polypeptide (HPP), a newly postulated hormone, was clearly associated with a fourth cell type, which is characterized by the presence of small secretory granules (100-150 nm.). These results suggest that each of the four cell types that are easily identifiable by ultrastructural observations is responsible for the production of a specific secretory product.