The effect of carbon source on a number of cultural parameters was investigated in Streptomyces thermoviolaceus. Glucose-grown cultures produced the antibiotic granaticin during pH-controlled growth in a fermenter. Biphasic growth occurred for all the carbon sources tested at 45-degrees-C, the inflexion of which occurred at a biomass density of between 0.5 and 0.6 gL-1 and which coincided with the onset of appearance of secondary metabolites. Maximum antibiotic production occurred in proline-grown cultures, which also had the slowest growth rates during the secondary phase of growth. Respiratory chain activity was probed by measuring NADH oxidation in membrane preparations exposed to a range of cyanide concentrations. Modulation of the terminal oxidase activity was apparent so that membranes prepared from cultures in the antibiotic-producing phase were less sensitive to KCN than those prepared from the early exponential phase of growth. The probable reason for this difference was the synthesis of cytochrome oxidase d during later stages of growth. These changes in respiratory activity are discussed in relation to patterns of growth and timing of the appearance of secondary metabolites synthesised by S. thermoviolaceus.