The biosynthesis of the antifungal, antiviral antiobiotic, sinefungin (1), produced by Streptomyces griseolus has been investigated. Precursor incorporation studies using (U-\sp14C), (5-\sp3H), (5-\sp13C) and (5-\sp13C, 5-\sp15N)-ornithine have shown that carbons 6\sp′-10\sp′ were derived from the intact incorporation of ornithine with retention of the C-5 nitrogen and with the loss of one of the protons from C-5 of ornithine. The results of the feeding experiments of (\sp14C) and (\sp3H) labelled adenosine have indicated that the adenine moiety of adenosine was incorporated intact. Administration of adenosine labelled on the ribose moiety indicated that adenosine was incorporated into sinefungin with 50% loss of protons from C-5\sp′ of adenosine.