In beta -lactam producing microorganisms, the first step in the biosynthesis of the beta -lactam ring is the condensation of three amino acid precursors: alpha -aminoadipate, L-Cysteine and D-valine. In Nocardia lactamdurans and other cephamycin-producing actinomycetes, alpha -aminoadipate is generated from L-lysine by two sequential enzymatic steps. The first step involves a lysine6-aminotransferase activity (LAT), considered to be one of the rate-limiting steps for antibiotic biosynthesis. Here, we report the effect of exogenous lysine on antibiotic production by N. lactamdurans MA4213. Lysine-supplemented cultures showed higher titers of cephamycin C. an effect that was more significant at early fermentation times. The increase in cephamycin C production was not quantitatively correlated with specific LAT activity in lysine-supplemented cultures. Observation of a positive effect of lysine on cephamycin C production by N. lactamdurans was dependent on carbon source availability in the culture media. Supplementation of the culture media with exogenous lysine did not affect the mRNA levels of the early biosynthetic genes controlled by the bidirectional promoter. These results indicate that L-lysine is required not only for antibiotic biosynthesis, but particularly as carbon or nitrogen source.