Previously we demonstrated that hamamelitannin (2', 5-di-O-galloyl hamamelose) in Hamamelis virginiana L. exhibits potent superoxide-anion scavenging activity. We then examined the physiological and pharmacological activities of hamamelitannin as well as its functional homologues, gallic acid and syringic acid. The following results were obtained : (1) Hamamelitannin has a higher protective activity against cell damages induced by superoxide anions than gallic acid which is the functional moiety of hamamelitannin. The protective activity of hamamelitannin on murine fibroblast-damage induced by superoxide anions was found at a minimum concentration of 50 μM, while the corresponding figure for gallic acid was 100 μM. (2) Pre-treatment of fibroblasts with hamamelitannin enhances cell survival. (3) The superoxide-anion scavenging activity of the compound in terms of its IC₅₀ value (50% inhibition concentration of superoxide anion radicals generated) was evaluated by ESR spin-trapping. Both hamamelitannin (IC₅₀=1.31±0.06 μM) and gallic acid (IC₅₀=1.01±0.03 μM) exhibited high superoxide-anion scavenging activity followed by syringic acid (IC₅₀=13.90±2.38 μM). (4) When hamamelitannin was treated with superoxide anions generated by a KO₂-crown ether system, HPLC analysis showed the disappearance of hamamelitannin and the concomitant formation of hamamelitannin-derived radicals (g=2.005, ΔH₁=2.16 G, ΔH₂=4.69 G) was detected by ESR spectrometry. From these observations, we concluded that (i) a compound with polyphenolic hydroxyl groups, especially the galloyl group (3, 4, 5-trihydroxy-), has potent scavenging activity against superoxide anions, and (ii) hamamelitannin is superior to gallic acid in protecting against cell damage induced by superoxide anions, suggesting that the high affinity of hamamelitannin for cells or membranes may be an important factor for protecting cells against active oxygen species.