Transformation of 1,2,3- and 1,2,4-trichlorobenzene in the presence of 20 mM sodium formate, by the methanotrophic bacterium Methylosinus trichosporium OB3b, was studied using cells grown in batch and continuous culture. Only 1,2,3-trichlorobenzene was transformed and transformation was strictly cometabolic, only catalysed in the presence of the soluble form of methane monooxygenase. The kinetics of transformation could be described by simple first-order kinetics (0.00193 l min(-1) g(-1)). Also the kinetics of transformation were found to be linearly proportional to cell density. No chloride ion release was observed during the reaction and the products of transformation (2,3,4- and 3,4,5-trichlorophenol) were identified by gas chromatography/mass spectroscopy and H-1-NMR and a 1.84:1 ratio of products in favour of pam hydroxylation was observed. It was also observed that the relationship between mass of substrate transformed and cell density was linear giving a transformation capacity of 88.8 +/- 11.8 mu mol g(-1), after which the transformation of 1,2,3-trichlorobenzene was inhibited. This inhibition was not due to O-2 limitation, co-substrate (CHOONa) limitation or product inhibition. Recovery and washing of the cells did not reverse this inhibition, indicating that inhibition was irreversible. During transformation a substantial decrease in the endogenous and formate-dependent O-2 consumption rates was observed, although the methanol-dependent O-2 consumption rate varied little between fresh cell samples and samples that had been used to transform 1,2,3-trichlorobenzene.