Flavonoids represent a diverse group of phytochemicals which possess the capacity to act as antioxidants in vitro. This study examined the free radical scavenging properties of a luteolin-rich artichoke extract and some of its pure flavonoid constituents by assessing their ability to prevent Cu2+-mediated LDL oxidation. Artichoke extract retarded LDL oxidation in a dose-dependent manner as measured by a prolongation of the lag phase to conjugated diene formation, a decrease in the rate of propagation and a sparing of endogenous LDL alpha-tocopherol during oxidation. The pure aglycone, luteolin (1 microM), demonstrated an efficacy similar to that of 20 microg/ml artichoke extract in inhibiting lipid peroxidation. Luteolin-7-O-glucoside, one of the glycosylated forms in the diet, also demonstrated a dose-dependent reduction of LDL oxidation that was less effective than that of luteolin. Studies of the copper-chelating properties of luteolin-7-O-glucoside and luteolin suggest a potential role for chelation in the antioxidative effects of artichoke extract. Overall, the results demonstrate that the antioxidant activity of the artichoke extract relates in part to its constituent flavonoids which act as hydrogen donors and metal ion chelators, and the effectiveness is further influenced by their partitioning between aqueous and lipophilic phases.