Ethanol extract of prune was separated into hexane-soluble and H(2)O-soluble fractions, and the H(2)O-soluble fraction was further separated into a methanol (MeOH) eluate and an H(2)O eluate by Diaion HP-20 column chromatography. The MeOH eluate exhibited the strongest antioxidant activity among the separated fractions evaluated by oxygen radical absorbance capacity (ORAC). Further purification of the MeOH eluate led to isolation of a novel compound, 4-amino-4-carboxychroman-2-one, together with four known compounds (p-coumaric acid, vanillic acid beta-glucoside, protocatechuic acid, and caffeic acid), structures of which were identified by NMR and MS analyses. The ORAC values of these isolated compounds showed 0.15-1.43 units (micromol of Trolox equiv)/micromol, and the new compound showed a remarkable synergistic effect on caffeoylquinic acid isomers. The antioxidant activity of the MeOH eluate was highly dependent on the major prune components, caffeoylquinic acid isomers, with a contribution from the new synergist.