The biosynthesis of the Cl4-labeled benzyipenicillin was achieved by preparing phenylacetic acid-1-C14 and incorporating it into the side-chain of benzyipenicillin. The antibiotic was successfully separated from the nonreacted labeled phenylacetic acid by column chromatography. The data obtained by radioauto-graphy and chemical degradation of the purified antibiotic indicated that the radioactivity resided in the side-chain of the benzyipenicillin molecule. In addition, phenylacetic acid was found to be decarboxylated and also incorporated into the protein and non-saponifiable lipid fractions of the mycelium.