The 90 kDa heat shock protein (hsp90) is a major cytoplasmic molecular chaperone associating with various other proteins such as steroid receptors, protein kinases and filamentous actin. hsp90 has also been shown to bind ATP, which causes a conformational change of the protein. The physiological role and significance of ATP binding by hsp90, however, has remained unclear. Here we show through direct, microscopic observations, that ATP induces the dissociation of actin filaments from immobilized molecules of hsp90 as well as the dissociation of F-actin from heavy meromyosin in the presence of hsp90.