Endocrine cells in 24 primary carcinomas and in the nonneoplastic mucosa of the small intestine were investigated histochemically and immunohistochemically with antisera against serotonin and 10 kinds of peptide hormones. Argyrophil-positive endocrine cells were found in four of eight duodenal, all of eight jejunal, and six of eight ileal carcinomas. The density of the positive cells was higher in the ileal tumors than in the duodenal and jejunal ones. Immunoreactive endocrine cells were detected in three duodenal, six jejunal, and five ileal carcinomas. Immunoreactive serotonin cells were present most frequently and most densely without respect to the site of origin of the carcinomas. In general, the population of endocrine cells among the tumor cells was higher in the ileal carcinomas than in the duodenal and jejunal ones. The ileal carcinomas frequently and densely demonstrated somatostatin, peptide YY, neurotensin, glucagon, and glicentin cells in addition to serotonin cells. The kinds of endocrine cells and the relative frequency of each kind of endocrine cell in carcinomas of the small intestine were similar to those in the nonneoplastic mucosa from which the carcinomas originated. This is the first systematic immunohistochemical study on endocrine cells in carcinoma of the small intestine.