The 90-kDa heat-shock protein (Hsp90) is a molecular chaperone that aids the folding of nuclear hormone receptors and protein kinases. Hsp90 x protein complexes can be stabilized by molybdate and by other transition metal oxyanions such as vanadate. Our earlier findings [Csermely, P., Kajtár, J., Hollósi, M., Jalsovszky, G., Holly, S., Kahn, C. R., Gergely, P. Jr, Söti, C., Mihály, K. & Somogyi, J. (1993) J. Biol. Chem. 268, 1901-1907] showed that vanadate and molybdate can induce a large conformational change of Hsp90. Here we provide direct evidence for the binding of vanadate and molybdate to Hsp90 by demonstrating that surface-plasmon-resonance measurements indicate binding of various vanadate oligomers to Hsp90. 51V-NMR measurements show an extensive interaction of decavanadate with the chaperone, and permolybdate treatment of Hsp90 induces a marked mobility shift of the protein and its tryptic fragments. Our results indicate the flexibility of molybdate/vanadate-binding sites of Hsp90, which are able to accommodate various species of these transition metal anions.