The origin of the antioxidant activity of Luobuma aqueous extract was examined by measuring the peroxynitrite (ONOO-)-eliminating activities of fractions of this extract obtained by Sephadex LH-20 column chromatography. Three of the four fractions obtained, i.e., those excluding the H2O-eluted fraction, were found to possess (ONOO-)-eliminating activity. These three fractions were combined and fractionated again by Sephadex LH-20 column chromatography, which yielded five fractions. Seven different compounds were isolated from two of these five fractions with high activity. Epigallocatechin-(4beta-8)-epicatechin showed the highest (ONOO-)-eliminating activity.