The objectives of this study were to investigate the radical-scavenging activity and protective potential of Sophora flavescens from oxidative damage by the radical generator 2,2'-azobis(2-amidinopropane)dihydrochloride (AAPH) in renal epithelial LLC-PK(1) cells and to identify the active components using the bioassay-linked fractionation method. The MeOH extract and fractions of CH(2)Cl(2), BuOH, and H(2)O from S. flavescens showed 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging effects in a dose-dependent manner (p<0.01),whereas only the BuOH and CH(2)Cl(2) fractions showed protective effects against LLC-PK(1) cellular damage induced by AAPH in a dose-dependent manner (p<0.01). In particular, the BuOH fraction had the most effective (p<0.05) antioxidative capacity. Employing a bioassay-linked HPLC/MS method, the active constituents from the BuOH fraction of S. flavescens were isolated and characterized as sophoraflavanone G and kurarinone with potent antioxidant effects against the DPPH radical, with IC(50) values of 5.26 and 7.73 microg/ml, respectively. Moreover, the compounds dose dependently recovered cell viability decreased by AAPH treatment (p<0.01), suggesting their protective roles against cellular oxidative damage. The results of this study suggest that S. flavescens has excellent antioxidative and kidney-protective potential and that flavonoids from S. flavescens, i.e., sophoraflavanone G and kurarinone, are the active constituents.