Experiments with radioactive precursors showed that the terminal group of tetracyclines can originate by various pathways. Both malonylCoA originating by carboxylation of acetylCoA, and malonylCoA synthesized by alternative biosynthetic pathways, can be utilized. Radioactive malonate semiamide or its derivative was hydrolyzed to malonic acid during the fermentation and the acid was incorporated into tetracycline in the normal way. Of the other assumed precursors, U-14C-uracil, 1-14C-propionate, U-14C-alanine, 5-14C-glutamate were unspecifically incorporated into the terminal group of tetracycline. The incorporation of 4-14C-ethyl succinamate and 5-14C-ethyl glutaramate was negligible.