Antioxidative effect of (-)-epicatechin,(-)-epicatechin gallate, and quercetin was examined by measuring the inhibition of lipid peroxidation in large unilamellar liposomes composed of egg yolk phosphatidylcholine (PC). These catechol-type flavonoids were stable in the liposomal suspension. They retarded the accumulation of PC-hydroperoxides depending on their concentrations when the suspension was exposed to an water-soluble radical initiator, 2,2'-azobis(2-amidinopropane)hydrochloride (AAPH). Their inhibitory effects lasted longer than that of alpha-tocopherol. When each flavonoid and alpha-tocopherol were mixed in the liposomes, epicatechin and epicatechin gallate disappeared in favor of alpha-tocopherol. Quercetin also decreased faster than alpha-tocopherol in the initial stage of incubation. Kinetic studies of the inhibition of radical chain oxidation of methyl linoleate in solution demonstrated that the rate constants for the inhibition of oxidation by these flavonoids (kinh) were 5-20 times smaller than that by alpha-tocopherol. It is likely that the flavonoids are localized near the surface of phospholipid bilayers suitable for scavenging aqueous oxygen radicals and thereby they prevent the consumption of lipophilic alpha-to-copherol. Epicatechin and epicatecin gallate gave smaller kinh values than quercetin. Voltammograms of these compounds showed that electron-donating ability of catechins was lower than that of quercetin. However, antioxidative effects of catechins were comparable to that of quercetin in AAPH-initiated peroxidation of the liposomal suspension. It is concluded that catechins and quercetin serve as powerful antioxidants against lipid peroxidation when phospholipid bilayers are exposed to aqueous oxygen radicals.