Active oxygen species, e. g., hydrogen peroxide (H2O2), superoxide anion radical (O2-), hydroxyl radical (·OH) and lipid hydroperoxides, have been implicated as mediators of oxidative cell injuries in a variety of clinical conditions. Some metabolites of carcinogenic compounds, cigarette smoke and glucosone, an intermediate of the Maillard reaction, were found to produce H2O2 in their autoxidation processes, and to induce both cytotoxicity and DNA single-strand breaks in mammalian cells. Inhibition of these cell injuries by catalase indicated that H2O2 was the critical species. In contrast to the above prooxidant effects, antioxidant effects of food ingredients on H2O2-induced cytotoxicity were also investigated. Polyphenols, e. g., caffeic acid ester and flavonoids, were effective for suppression of H2O2-induced cytotoxicity on mammalian cells. A study of the structure-activity relationship revealed that the affinity of polyphenols for cell membranes and the presence of an ortho-dihydroxy moiety in their structure were essential for their protective action.