It has been suggested that reactive oxygen intermediates (ROIs) may have a role in the genotoxic effects of lead (Pb2+) and mercury (Hg2+), but there have not been any definitive studies demonstrating a causal relationship between the induction of ROIs by these metals and mutagenesis. We previously demonstrated, using the transgenic Chinese hamster ovary cell line AS52, that low concentrations (0.1-1 microM) of Pb2+ and Hg2+ are mutagenic. In the present study, using a novel histochemical computer-enhanced image analysis technique, we demonstrate that Pb2+ and Hg2+ induce the formation of H2O2 in AS52 cells by at least two distinct mechanisms. One is characterized by the rapid induction of H2O2 following treatment of cells with concentrations of Pb2+ or Hg2+ below 0.8 and 1 microM, respectively, while the second occurs in AS52 cells treated with concentrations of Pb2+ or Hg2+ greater than 0.8 and 1 microM, respectively. Pb2+ and Hg2+ (0.1-1 microM) had no effect on the activities of partially purified catalase, glutathione peroxidase, or glutathione reductase, important enzymes involved with antioxidant defense, but these metals stimulated the activities of copper-zinc superoxide dismutase (CuZn-SOD) and xanthine oxidase (XO). Allopurinol (50 microM), a specific inhibitor of xanthine oxidase, inhibited the induction of H2O2 by Pb2+ (0.8-1 microM) and Hg2+ (1 microM) and also inhibited Pb2+- and Hg2+-induced mutagenesis. These results demonstrate that Pb2+ and Hg2+ disrupt the redox status of AS52 cells by enhancing the activities of CuZn-SOD and XO. Furthermore, the results of these studies also demonstrate that there is a causal relationship between the induction of H2O2 by these metals and mutagenesis.