The crude MeOH extract prepared from the aerial parts of G. stepporum and its EtOAc, n-BuOH, H(2)O subextracts were screened for their inhibitory activity on H(2)O(2)-induced lipid peroxidation in human red blood cells. The EtOAc subextract was found to be the most active one and its chemical composition was further analysed. Six polyphenolic secondary metabolites including quercetin (1), rutin (2), methyl gallate (3), pusilagin (4), 1,3,6-tri-O-galloyl-beta-glucospyranose (5) and 1,2,3,4,6-penta-O-galloyl-beta-glucopyranose (6) were isolated from the EtOAc extract by successive chromatographic methods (Sephadex CC, silica gel CC and P-TLC). The structures of the isolates were elucidated by 1D- and 2D-NMR and MS. This is the first report of occurrence of hydrolysable tannins in G. stepporum.