The objective of this study was to test the hypothesis that hepatocarcinogenesis by peroxisome proliferators, a novel class of chemical carcinogens, is mediated either directly by carcinogenic H₂O₂, generated by peroxisomal oxidase(s) or indirectly by free radicals produced from H₂O₂, and that antioxidants could retard or inhibit neoplasia by scavenging active oxygen (superoxide radicals O₂^-, hydrogen peroxide, hydroxyl radicals HO, and singlet oxygen ¹O₂). Accordingly, the effect of synthetic antioxidants 2(3)-tert-butyl-14-hydroxyanisole and ethoxyquin on the peroxisome proliferator 2-[4-(2,2-dichlorocyclopropyl)phenoxy]2-methyl-propionic acid (ciprofibrate)-induced hepatic tumorigenesis has been examined in male Fischer 344 rats. Rats were fed either a 2(3)-tert-butyl-4-hydroxyanisole (0.5% w/w)- or ethoxyquin (0.5% w/w)-containing diet with or without ciprofibrate (10 mg/kg of body weight) for 60 weeks. Rats fed ciprofibrate (10 mg/kg of body weight) in the diet or fed a diet with no added chemicals served as controls. Results of this study demonstrated that ethoxyquin markedly inhibited the hepatic tumorigenic effect of ciprofibrate, as evidenced by a decreased incidence of tumors, a decreased number of tumors per liver, and a reduced tumor size. 2(3)-tert-Butyl-4-hydroxyanisole also caused a significant decrease in the incidence and number of hepatocellular carcinomas that were larger than 5 mm. The present data suggest that the inhibitory effect of antioxidants on ciprofibrate-induced hepatic tumorigenesis may be due to H₂O₂ and free radical-scavenging property of ethoxyquin and 2(3)-tert-butyl-4-hydroxyanisole, since these antioxidants do not prevent peroxisome proliferation and induction of H₂O₂-generating peroxisomal enzymes in livers of rats fed ciprofibrate. Whether the inhibitory effect of antioxidants is exercised on the presumptive H₂O₂ initiation process and/or on the postinitiation growth phase of foci and nodules in liver is, at present, unknown.