Tea catechins, (-)-epigallocatechin-3-gallate (EGCg) and (-)-epigallocatechin (EGC), have been reported to suppress oxidation of plasma low density lipoprotein (LDL) in vitro. If dietary catechins can be efficiently incorporated into human blood plasma, anti-atherosclerotic effects in preventing oxidative modification of LDL would be expected. In this study, a newly developed chemiluminescence detection-high pressure liquid chromatography (CL-HPLC) method for measuring plasma catechins was used and the incorporation of EGCg and EGC into human plasma was investigated. Healthy subjects orally ingested 3, 5, or 7 capsules of green tea extract (corresponding to 225, 375, and 525 mg EGCg and 7.5, 12.5, and 17.5 mg EGC, respectively). The plasma EGCg and EGC concentrations before the administration were all below the detection limit (< 2 pmol/ml), but 90 min after, significantly and dose-dependently increased to 657, 4300, and 4410 pmol EGCg/ml, and 35, 144, and 255 pmol EGC/ml, in the subjects who received 3, 5, and 7 capsules, respectively. Both EGCg and EGC levels detected in plasma corresponded to 0.2-2.0% of the ingested amount. Catechin intake had no effect on the basal level of endogenous antioxidants (alpha-tocopherol, beta-carotene, and lycopene) or of lipids in plasma. These results suggested that drinking green tea daily would contribute to maintain plasma catechin levels sufficient to exert antioxidant activity against oxidative modification of lipoproteins in blood circulation systems.