Summarizing, a catechol structure for enterochromaffin seems more indicated than a hydroquinone structure. Any aminophenol struture, primary or secondary, seems contraindicated. The resorcinol hypothesis seems untenable. Serotonin is chemically and sometimes histochemically demonstrable in carcinoid tumors, but its presence in normal enterochromaffin cells in demonstrable amounts seems contraindicated. The azo positive phenol in the enterochromaffin cells appears to be a distinct substance.