In recent years many studies have been directed toward the elucidation of the interaction mechanisms within protein-protein complexes. One of the best studies protein-protein complexes has been the cytochrome b5 and cytochrome c electron transfer pair. Thermodynamic information about the association process has been obtained through methods which indirectly measure the binding between the proteins. We report here the use of Isothermal Titration Calorimetry to characterize the association of Rat cytochrome b5 and Horse cytochrome c. The association is accompanied by an unfavorable enthalpy change (+1.0 +/-0.1 Kcal/mole) and a large stabilizing change in entropy (33.9 +/-0.6 eu).