Flavonoids are reported to exhibit a wide variety of biological effects, including antioxidant and free radical-scavenging activities. Reactive oxygen species have been implicated in a range of human pathological diseases such as atherosclerosis and certain cancers. The aims of this present study were 1) to investigate the effect of the flavonoids myricetin, quercetin, and rutin on cell viability, endogenous antioxidant enzyme activities, and DNA integrity in Caco-2 and Hep G2 cells and 2) to determine whether these flavonoids could protect against H2O2-induced DNA damage. Both cell lines were supplemented with various concentrations (0-200 mu M) of myricetin, quercetin, and rutin for 24 hours or H2O2 (50 mu M) for 30 minutes, and cell viability was assessed. Over the concentration range tested neither the flavonoids nor H2O2 significantly affected cell viability. The effect of the flavonoids on the activities of the antioxidant enzymes catalase (EC 184.108.40.206) and superoxide dismutase (EC 220.127.116.11) and on DNA integrity was assessed. The flavonoids did not significantly affect catalase or superoxide dismutase activity and did not induce DNA damage in either cell line. Exposure to 50 mu M H2O2 for 30 minutes at 37 degrees C resulted in significant DNA damage, and preincubation with the flavonoids before H2O2 exposure significantly (p < 0.05) protected Caco-2 and Hep G2 cells against H2O2-induced DNA damage.