As other nuclear receptors, steroid hormone receptors form large protein hetero-complexes in their inactive, ligand-friendly state. These receptors are only stable in it's unliganded form when associated with accompanying proteins. Several heat shock proteins, immunophilins, and others have been identified as members of these highly dynamic complexes. The interaction kinetics and dynamics of hsp90, hsp70, p60 (hop), FKBP52, FKBP51, p48 (Hip), and p23 have been assessed by a biosensor approach measuring the complex formation in real time. As core chaperon complex p60, hsp90 and hsp70 has been reconstituted. p60 forms a molecular bridge between hsp90 and hsp70 with an affinity in the range 105 M-1. Dynamics of hsp90-p60 complex formation is modulated by ATP through changes in the co-operativity of the interaction. At low protein concentrations ATP is stabilizing the complex. Binding of p23 to hsp90 did not change the affinity of the hsp90-p60 complex and the stabilizing effect of ATP. Saturation of p48-hsp70 interaction could not be achieved suggesting multiple biding sites. A picture of the protein complex including stoichiometric coefficients, co-operativity of interaction and equilibrium binding constants have been evolved.