Intestinal mucosal biopsy specimens processed during the past 25 years were used to example the ultrastructural characteristics of intestinal endocrine cells. The cells were defined on the basis of morphologic criteria and, when feasible, with specific antisera and immunogold staining. The hypothesis was that each endocrine cell, once well defined, should be identifiable on the basis of standard morphologic criteria not requiring specific immunostaining. This was not the case, D, G, EC1, EC2, ECn, D1, and intestinal gastrin cells have characteristic secretory granules and, when sufficient granules are present, can be identified consistently on the basis of morphologic criteria. Absolute identification of D, G, IG, and TG cells requires staining with specific antisera, a condition easily obtainable only for D, G, and IG cells. D1, EC1, EC2, and ECn cells must be identified morphologically until secretory products specific for each of these cells are identified. I, L, N, and K cells are remarkably similar in appearance and must be distinguished by specific staining. Mo, S, and P cells were not identified by either morphologic appearance or immunostaining. It is suggested that a cell similar to the D1 cell but with exceptionally small granules may be the P cell. Absolute identification of intestinal enteroendocrine cells by electron microscopy requires specific staining. The characteristic appearance of the secretory granules of many of these cells (D, G, EC1, EC2, ECn, D1, and IG) permits morphologic identification when numerous secretory granules are present.