Although gastric and intestinal contents from rats failed to show amino acid decarboxylase activity when tested against five different amino acids (glutamic acid, arginine, lysine, tyrosine, and histidine), the feces contained at least seven different amines, some known to be pharmacologically active. Putrescine, histamine, and tyramine were identified by means of paper chromatography in both intestinal material and mixed fecal cultures; four other spots were found, three of which had Rf values similar to agmatine, ethanolamine, and ephedrine. The formation of lysine and glutamic acid decarboxylases was not enhanced by an increased acidity during growth while increased oxygen tension was inhibitory to amino acid decarboxylase synthesis in these fecal cultures. The feeding of chlortetracycline to rats, or its presence at a very low concentration in media in which the mixed cultures were grown, reduced the capacity of intestinal microorganisms to produce amines. Cells from mixed fecal cultures grown in the presence of chlortetracycline lacked or contained but weak amino acid decarboxylase activities. The action of the enzymes themselves was unaffected by the presence of the antibiotic in the Warburg cup during assay. The results suggest that amines formed within the intestinal tract might be toxic to the rat, and that chlortetracycline accelerates animal growth by suppressing their production.