Inductions of oxidative DNA damage (oh8dG) in vitro and peritoneal mesothelioma in rats (F344, female) were compared between crocidolite (CR) and de-ironized crocidolite [DCR, washed by HCl and ethylenediamine tetraacetic acid (EDTA)] to verify the hypothesis that reactive oxygen species contribute to carcinogenesis, focusing on the role of iron present inside or outside of the CR. The yield of oh8dG was 14.6 oh8dG/10(5)dG in CR and 30.2 in DCR under simple incubation with DNA. In the incubation systems added several chemicals and H2O2, DCR induced higher levels of oh8dG than CR. Especially, the addition of Fe2O3 and H2O2 to DCR increased oh8dG in DNA depending on the Fe2O3 concentration, however, this tendency was not observed in the same system of CR. Surprisingly, 7 out of 10 rats died within 2 days after the injection of 10 mg of Fe2O3 following the DCR injection (5 mg/rat), showing necroses of hepatocytes from the surface of each lobe where CR and Fe2O3 particles had been deposited together. There was no death in other groups of rats. One year after the i.p. injection of CR (5 mg/rat, single injection), mesotheliomas were found in all rats administered DCR and Fe2O3 (2 mg/rat, once a week, for 35 weeks), in 4 rats of DCR alone (n = 10), in 5 rats of CR alone (n = 10) and in none of the rats administered Fe2O3 alone (n = 10). Therefore, present results indicate that the induction of oxidative DNA damage changed even when the same type of asbestos was washed by chemical treatment, and Fe2O3 promoted the development of mesothelioma which was induced by DCR.