Sphingomonas chlorophenolica RA-2 is a soil microorganism that can grow on pentachlorophenol (PCP) as a sole carbon source. In this microorganism, PCP is converted to tetrachlorohydroquinone (TCHQ), trichlorohydroquinone, and 2,6-dichlorohydroquinone. The remainder of the pathway has not yet been defined. The ability to grow on PCP as a sole carbon source is remarkable because of the toxicity of PCP and its chlorinated hydroquinone metabolites. Experiments in which the levels of PCP and chlorinated hydroquinones were measured in cells metabolizing [U-14C]PCP revealed that the levels of chlorinated hydroquinones in the cytoplasm are in the low micromolar range. The toxicity of chlorinated hydroquinones was evaluated by exposure of Escherichia coli cells that had been treated with EDTA (to remove the outer membrane) to TCHQ. Significant toxicity due to TCHQ was not apparent until concentrations of 500 microM and higher. Thus, an important part of the explanation for why S. chlorophenolica RA-2 is able to grow on PCP as a sole carbon source is undoubtedly that it can process sufficient carbon for growth without accumulating high levels of toxic intermediates.