The broad range of substrate polypeptides that interact with the GroEL chaperonin has prompted the proposal that hydrophobic interactions provide the primary force for GroEL-polypeptide interactions. One measure of the relative hydrophobicity of nonnative proteins is the commonly used 'm-value' determined from denaturation studies, which correlates with the extent of exposed hydrophobic surface area of the denatured state. In an effort to assess the importance of hydrophobic surface in a series of similar chaperonin substrates, the interaction of several variants of Staphylococcal nuclease with GroEL has been investigated. The experimental design is to undertake binding studies with GroEL using nuclease variants that have the same free energy for unfolding, but which possess different extents of exposed hydrophobic surface in their denatured states to characterize the chaperonin recognition process. The interaction energetics of Staphylococcal nuclease variants with GroEL has been measured by isothermal titration calorimetry and by fluorescence. Initial results with several nuclease variants indicate that binding occurs with a positive enthalpy change, and is therefore entropically controlled at room temperature. Interestingly, the temperature dependence of the enthalpy change for binding yields a positive change in the heat capacity. Only a very weak correlation is seen between the affinity for GroEL and the denaturation parameters for the variants.