A complete microcalorimetric investigation of the interactions between the native subunits of the bovine pancreatic procarboxypeptidase A-S6 ternary complex has been performed. All the association constants and thermodynamic parameters associated with the reactions forming the various complexes have been determined. The influence of pH and ionic strength on the binding reactions has been investigated. Interestingly, the affinity between the subunits is not significantly modified by varying the ionic strength. In this respect, an enthalpy/entropy compensatory effect is observed for the binding of subunit III to subunit I when the ionic strength is increased, suggesting a physiological function for the association. The various pathways for formation of the ternary complex have been studied. Binding of subunit II (or III) to subunit I, the central element of the ternary complex, does not significantly modify the affinity of the other subunit for subunit I. From a thermodynamic point of view, the same final state is obtained whatever the pathway of ternary complex formation. This study is the first step of a kinetic investigation of the associated subunits.