A catechol structure for enterochromaffin seems more indicated than a hydroquinone structure. Any aminophenol structure, primary or secondary, seems contra-indicated. 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.