Abstract: Objective 　To investigate the effects of NB on Bcr-Abl expressing K562 cells, and the relationship between these effect s and the molecular chaperone functions of heat shock protein 90 ( Hsp90 ) . Methods 　The amount s of Bcr-Abl protein were tested by Western blot . Molecular chaperone functions of Hsp90 were measured by co-immunoprecipitation. Co-immunoprecipitation of Bcr-Abl and its molecular chaperones, the immunoprecipitate was then subjected to Western blot analysis with anti-Abl, antiHsp90, or anti-Hsp70 mAb. Results 　Exposure of K562 cells to NB produced down-regulation of int racellular Bcr-Abl protein levels. By binding and inhibiting Hsp90, NB treatment decreased the binding of Bcr-Abl with Hsp90 and Hsp70. Conclusion 　These studies demonst rate for the first time the activity of NB against CML in vitro involves destruction of Hsp90 chaperon function.