The role of clavulanic acid, an unstable antibiotic produced by Streptomyces clavuligerus, in biomass accumulation and production of clavulanic acid in batch cultures of the organism was examined. The organism was grown in a medium containing either 20g/l lysine, 1g/l lysine or 1g/l lysine supplemented with degraded clavulanic acid as nitrogen sources. Biomass accumulation was highest in cultures grown with supplemented degraded clavulanic acid and reached a maximum of 2.2g/l, compared with 1.5g/l when lysine only was used. The yield coefficient for clavulanic acid production was again highest in cultures grown with supplemented degraded clavulanic acid, with a Y p / x value of 2mg/g compared with Y p / x value of 1.5mg/g in 20g/l lysine. No clavulanic acid was produced in cultures containing non-supplemented 1g/l lysine. Non-degraded clavulanic acid was added at 60h to non-producing cultures of the organism containing 1g/l lysine only. Clavulanic acid concentration immediately decreased on addition from 0.04g/l over a period of 20h, then remained constant at 0.02g/l for a further 30h until the end of the cultivation. This suggests that the rate of degradation was equivalent to the rate of production of clavulanic acid following a period of initial additive degradation. These results indicate that clavulanic acid is both produced and degraded in cultures of S. clavuligerus and that the products of degradation are used by the organism, resulting in further production of the antibiotic.