OBJECTIVE: To explore the antitumor activities of kushen (Sophora flavescens) flavonoids (KS-Fs) in vivo and in vitro. METHODS: Cell proliferation was assayed by using methyl thiazolyl tetrazolium (MTT) method. H22 hepatocellular carcinoma and S180 sarcoma were induced in ICR mice. Lewis lung carcinoma was induced in C57BL/6 mice. H460 and Eca-109 tumor were induced in Balb/c nude mice by injecting 5x10(5) or 5x10(6) tumor cells in the right flank, respectively. RESULTS: KS-Fs could inhibit the growth of a variety of human tumor cell lines (A549, SPC-A-1, NCI-H460, etc.) in vitro. The antitumor efficacies were confirmed in the mice models of H22, S180 and Lewis lung tumors and the nude mice models of human H460 and Eca-109 xenografted tumors. The oral or intravenous maximum tolerated dose of KS-Fs was more than 2.8 g/kg or 750 mg/kg respectively, far more than the oral medial lethal dose of kushen alkaloids (< or = 1.18 g/kg). No adverse reactions were observed. CONCLUSION: These results suggest that KS-Fs or kurarinone may be developed as a novel antitumor agent.