<jats:p>A 7-alpha-methoxycephalosporin containing a carbamoyloxymethyl substituent at C-3 (cephamycin C) has been isolated from the extracellular fluid of an aqueous suspension of Streptomyces clavuligerus shaken in the presence of 18O2. The cephalosporin has been converted into its N-acetyl dimethyl ester and the distribution of 18O in the latter determined by chemical-ionization mass spectrometry. The results indicate that the oxygen atom of the methoxy group, as well as that linked to the exocyclic methylene group at C-3, is derived from molecular O2.