The effect of pentachloronitrobenzene (PCNB) on denitrification was assessed with two denitrifying cultures (PCNB-free control and PCNB-acclimated) developed from a contaminated estuarine sediment. PCNB was transformed to pentachloroaniline (PCA) in the PCNB-acclimated culture repeatedly amended with 0.1 mu M PCNB, but further dechlorination or degradation of PCA was not observed for almost 1 year. The effect of PCNB on denitrification was also investigated with the PCNB-free control culture. PCNB at an initial concentration of 13 mu M was transformed to PCA simultaneously with nitrate reduction but only after the nitrate concentration was at or below 20 mg N/l. PCNB addition at an initial concentration of 13 mu M to the control denitrifying culture developed as PCNB-free culture resulted in a transient accumulation of nitric oxide (NO) and nitrous oxide (N(2)O). Similarly to the PCNB-acclimated culture, PCNB transformation to PCA started when the nitrate concentration decreased to about 20 mg N/l. A low degree of nitro group removal resulting in the formation of pentachlorobenzene (PeCB) was also observed in the control culture when amended with 13 mu M PCNB. Further transformation or degradation of PCA was not observed in all cultures maintained under active nitrate reducing conditions. Based on the results of this study, the presence of nitrate at low concentrations in anoxic/anaerobic soil and sediments is not expected to negatively affect the biotransformation of PCNB to PCA, but dechlorination or degradation of PCA is not expected under active nitrate reducing conditions.