The effect of the concentration of a limiting substrate on growth and the concentrations of several intracellular components of recombinant E. coli JM 109 containing the plasmid pUC13 were investigated. The limiting substrate was glucose and the intracellular components measured were plasmid content, RNA content, and beta-lactamase activity. The specific growth rate increased with increase in substrate concentration up to 16 g/litre. A further increase in glucose concentration inhibited growth. The RNA content increased with increasing specific growth rate while the plasmid content and beta-lactamase activity decreased. An unstructured model for the effect of substrate concentration on the specific growth rate was found to fit the experimental data. The equation simulates substrate inhibition during growth at high glucose concentrations.