GroEL is known to retard the refolding of apo-alpha-lactalbumin by interacting with the molten globule state of the protein. In order to investigate the dominant forces in this interaction, the GroEL-affected kinetic refolding of apo-alpha-lactalbumin from its acidic molten globule state was studied at different temperatures and in the presence of different kinds of monovalent cations at a fixed temperature (25 degrees C), by stopped-flow fluorescence measurements. The binding constant between GroEL and alpha-lactalbumin in the molten globule state was evaluated quantitatively from the kinetic refolding curves in the absence and presence of GroEL. The binding was found to be entropy-driven at room temperature and the heat capacity change for the binding was found to be largely negative (-3.6 kJ mol-1.K-1), indicating that GroEL binds to alpha-lactalbumin through hydrophobic interactions. The study of the effect of different monovalent cations at various ionic strengths shows that the binding is strengthened by electrostatic screening by ions, demonstrating the importance of electrostatic interactions. The relationship of these results with a putative target recognition site of GroEL will be discussed.