From the screening of a unique collection of 880 off-patent small organic molecules, we have found that quinacrine inhibits the interaction between a BH3 domain-derived peptide and the antiapoptotic protein Bcl-xL. Nuclear magnetic resonance spectroscopy confirmed that quinacrine binds to the hydrophobic groove that Bcl-xL uses for interacting with the BH3 domain of proapoptotic proteins. This activity can contribute to the anticancer activity of quinacrine. © 2009 Elsevier Ltd. All rights reserved.