Cells without Bak and Bax are largely resistant to apoptosis (1;2), despite the presence of other key components of the apoptotic machinery. We screened 7,800 natural compounds and found several that could specifically induce caspase activation and the release of cytochrome c (cyto c) in the bak^-/-/bax^-/- cells. One of these was gossypol, a polyphenolic compound naturally found in cottonseed that has been used in antifertility trials. We found that gossypol, but not other Bcl-2-interacting molecules, induced cyto c release and loss of mitochondrial membrane potential (m) independently of mPTP and Bak/Bax activation. Furthermore, we found that gossypol induced an allosteric change in Bcl-2 in both bak^-/-/bax^-/- cells and Bcl-2 overexpressing cells. This change in Bcl-2 conformation led to the release of cyto c in the presence of Bcl-2 and Bcl-xL in reconstituted proteoliposomes. We also observed that gossypol substantially reduced the growth of tumor xenografts from Bcl-2 overexpressing cells in nude mice. We conclude that gossypol converts the antiapoptotic molecule Bcl-2 into a proapoptotic molecule that can mediate the release of cyto c and induce apoptosis--Lei, X., Chen, Y., Du, G., Yu, W., Wang, X., Qu, H., Xia, B., He, H., Mao, J., Zong, W., Liao, X., L., Mehrpour, M., Hao, X., Chen, Q. Gossypol induces Bax/Bak-independent activation of apoptosis and cytochrome c release via a conformational change in Bcl-2.