The abilities of green tea extract and its three major components to inhibit lipid peroxidation in low-density lipoprotein (LDL) catalyzed by copper were tested in vitro using malondialdehyde as a parameter of antioxidant activity. The results demonstrated that green tea extract markedly delays peroxidation with a dose-dependent pattern. Of the three components, polyphenols had the strongest action. Similar action was also shown in the theanine-treated group but was weaker than in the former, whereas caffeine had a very limited effect. Based on these data, it is concluded that green tea extract can effectively inhibit peroxidation and that this activity is due largely to the polyphenols it contains. According to the ultraviolet spectra, copper chelation is suggested to be one of the possible mechanisms of LDL antiperoxidation.