Several important signaling proteins including transcription factors and protein kinases depend on heat shock protein (Hsp)-90 for stability. p210^bcr-abl, a protein expressed in chronic myelogenous leukemia, is functionally inhibited by the benzoquinone ansamycin herbimycin A. Benzoquinone ansamycins also bind to and inhibit the activity of Hsp90. We now demonstrate that p210^bcr-abl is complexed with Hsp90 and its cochaperone p23 in K562 chronic myelogenous leukemia cells. Brief exposure to the benzoquinone ansamycin Hsp90 inhibitor geldanamycin (GA) decreases the association of p210^bcr-abl with Hsp90 and p23 and increases its association with the chaperones Hsp70 and p60^Hop. GA has a similar effect on chaperone association with v-src, another Hsp90-dependent oncogenic kinase. Loss of Hsp90/p23 association and acquisition of Hsp70/p60^Hop association of both p210^bcr-abl and v-src precede GA-induced degradation of these kinases. GA-induced degradation is mediated by the proteasome because proteasome inhibitors block the effects of GA, causing both p210^bcr-abl and v-src to accumulate in a detergent-insoluble cellular fraction. Both p210^bcr-abl and v-src are more susceptible to GA-induced degradation than are their normal cellular counterparts, c-abl and c-src.