The 90 kDa heat shock protein (hsp90) is a member of the "chaperone-complex" of steroid receptors believed to be partially or transiently localized in the cell nucleus. Demonstrating that hsp90 has an ATP binding site and autophosphorylating activity we have observed that histones, especially histone H1, are able to modulate the autophosphorylation of hsp90 [Csermely, P. and Kahn, C.R. (1991) J. Biol. Chem. 266, 4943-4950]. Our present data suggest a direct interaction of hsp90 with histones, showing that hsp90 is able to bind histone-agarose and enhances the binding of histones to DNA. Circular dichroism spectra of rat liver chromatin indicate that hsp90 induces a tighter, condensed state of the chromatin structure which is resistant against disruption by high salt treatment. Interactions of hsp90 with the chromatin may be important in regulating the transcriptional activity of steroid receptors and other transcription factors.