Grape seed polyphenols have been reported to exhibit a broad spectrum of biological properties. In this study, eleven phenolic phytochemicals from grape seeds were purified by gel chromatography and high performance liquid chromatography (HPLC). The antioxidant activities of five representative compounds with different structure type were assessed by the free radical-scavenging tests and the effects of the more potent phytochemicals on oxidative damage to DNA in mice spleen cells were investigated. Procyanidin B4, catechin, epicatechin and gallic acid reduced ferricyanide ion and scavenged the stable free radical, alpha, alpha-diphenyl-beta-picrylhydrazyl (DPPH) much more effectively than the known antioxidant vitamin ascorbic acid, while epicatechin lactone A, an oxidative derivative of epicatechin, did not reduce ferricyanide ion appreciably at concentrations used and was only about half as effective on free radical-scavenging as epicatechin. Mice spleen cells, when pre-incubated with relatively low concentration of procyanidin B4, catechin or gallic acid, were less susceptible to DNA damage induced by hydrogen peroxide (H2O2), as evaluated by the comet assay. In contrast, noticeable DNA damage was induced in mice spleen cells by incubating with higher concentration (150 microM) of catechin. Collectively, these data suggest that procyanidin B4, catechin, gallic acid were good antioxidants, at low concentration they could prevent oxidative damage to cellular DNA. But at higher concentration, these compounds may induce cellular DNA damage, taking catechin for example, which explained the irregularity of dose-effect relationship.