The nonspecific interaction between lead ions and bovine serum albumin (BSA) was studied by the calorimetric technique. According to thermodynamic parameters calculated from titration curves, it can be seen that the increase of intermolecular bond energies and decrease of disorder in the system were accompanied by a binding process. This kind of binding is the reaction "driven by enthalpy." Furthermore, the denatured BSA has more binding sites and more changes in enthalpy and entropy than the native BSA because the unfolded chain of denatured BSA could adapt itself to the binding reaction with lead ions more easily.