Butane monooxygenase (BMO) catalyses the oxidation of alkanes to alcohols in the alkane-utilizing bacterium 'Pseudomonas butanovora'. Incubation of alkane-grown 'P. butanovora' with butyrate or propionate led to irreversible time-and O 2 -dependent loss of BMO activity. In contrast, BMO activity was unaffected by incubation with lactate or acetate. Chloramphenicol inhibited the synthesis of new BMO, but did not change the kinetics of propionate-dependent BMO inactivation, suggesting that the propionate effect was not simply due to it acting as a repressor of BMO transcription. BMO was protected from propionate-dependent inactivation by the presence of its natural substrate, butane. Although both the time and O 2 dependency of propionate inactivation of BMO imply that propionate might be a suicide substrate, no evidence was obtained for BMO-dependent propionate consumption, or 14 C labelling of BMO polypeptides by [2-14 C]propionate during inactivation. Propionate-dependent BMO inactivation was also explored in mutant strains of 'P. butanovora' containing single amino acid substitutions in the a-subunit of the BMO hydroxylase. Propionate-dependent BMO inactivation in two mutant strains with amino acid substitutions close to the catalytic site differed from wild-type (one was more sensitive and the other less), providing further evidence that propionate-dependent inactivation involves interaction with the BMO catalytic site. A putative model is presented that might explain propionate-dependent inactivation of BMO when framed within the context of the catalytic cycle of the closely related enzyme, soluble methane monooxygenase.