In this report, the mechanism of the antitumor activities of Kushen flavonoids (KS-Fs) were explored. KS-Fs and kurarinone (Kur), a single flavonoid compound, were able to induce apoptosis of H460 and Eca-109 cells in vitro and H460 cells in vivo. The apoptosis inducing effect was enhanced in the presence of Taxol. In H460 xenograft mice treated with Kur, down-regulation of Bcl-2 and up-regulation of caspase 8 and caspase 3 in tumors were observed by immunohistochemical staining. In addition, KS-Fs and Kur were able to inhibit TNFalpha-induced NF-kappaB activation in 293 cells mediated by the decreased IkappaBalpha phosphorylation. Further the effects of KS-Fs and Kur on multiple receptor tyrosine kinase activities were explored. In cell-based assays, KS-Fs and Kur inhibited the EGF-induced EGF receptor phosphorylation in A431 cells and a constitutively activated Her-2 in MDA-MB-453s cells. In enzymatic assays, KS-Fs and Kur inhibited KDR, but not PDGF BR activities. In A431 xenograft mice treated with Kur, an inhibition of EGF receptor phosphorylation in tumors was observed. These results reveal a novel mechanism by which KS-Fs induces apoptosis in tumors by acting on multiple cellular targets including the inhibition of NF-kappaB activation and multiple receptor tyrosine kinase activities.