After addition of propionic acid-[1-14C-3-T] to cultures of Streptomyces erythreus the 14C/T ratio in the isolated erythromycin was determined and Kuhn -Roth oxidation was carried out to determine the location of the 14C and T activity in the molecule. The results prove that the lactone ring of erythromycin is derived from propionate units. Possibly propionic acid also plays a part in the biosynthesis of cladinose.