Hsp90 is a very abundant molecular chaperone that apparently helps to protect cellular proteins from denaturation upon temperature upshift. The unusual ability of Hsp90 to function under conditions where other proteins unfold prompted us to investigate the stability and structural organization of Hsp90 itself. Both procaryotic and eucaryotic members of the Hsp90 family were found to have very similar physicochemical properties: (i) they are stable against thermal unfolding up to at least 50 degrees C, (ii) they show biphasic, reversible unfolding transitions in guanidinium chloride, and (iii) their oligomerization state is strongly and rapidly affected by millimolar concentrations of divalent cations. In the presence of MnCl2 and MgCl2 defined changes in the quaternary structure of Hsp90 could be observed which resulted in a decrease in thermostability and an increased tendency to form larger aggregates. The addition of divalent cations also almost completely abolished the chaperone function of Hsp90 and induced release of folding intermediates of citrate synthase bound to Hsp90. These modulating effects of divalent cations on structure and function of Hsp90 in vitro represent a potential mechanism for regulation of Hsp90 chaperone action in vivo.