Occurrence and distribution of cytokeratin, neuron specific enolase (NSE) and actin were studied by the immunoperoxidase-antiperoxidase (PAP)-technique using specific antibodies in formalin-fixed, paraffin-embedded material from 6 cases of neurogenic appendicopathy with numerous endocrine cells in the mucosal stroma (SEC), 5 cases of microcarcinoidosis of the stomach, 12 gastrointestinal carcinoids and 4 bronchial carcinoids. Cytokeratin was detectable in all tumor cells. In addition, the epithelial endocrine cells (EEC) and the SEC of intestinal origin were cytokeratin positive. EEC, SEC and cells of microcarcinoids and carcinoids showed a positive immunoreactivity with antibodies to NSE, whereas actin antibodies did not reveal significant staining of these cells. These results strongly suggest that carcinoids of the gastrointestinal tract originate from SEC that have migrated downwards into the stroma from the epithelial layer (" Endophytie " according to Feyrter ).