Glycosaminoglycan (GAG)-protein binding governs critically important signaling events in living matter. Aiming at a quantitative analysis of the involved processes, we herein present a thermodynamic study of the interaction of the model GAG heparin and lysozyme in aqueous solution. Heparin is a highly charged linear polyelectrolyte with a charge parameter of 2.9 (37 degrees C). The binding constant K-b was determined by ITC as a function of the temperature and ionic strength adjusted through the concentration c(s) of added salt. The dependence on salt concentration c(s) was used to determine the net number of released counterions. Moreover, the binding constant at a reference salt concentration of I M K-b (1 M) was determined by extrapolation. The dependence on temperature of K-b was used to dissect the binding free energy Delta G(b) into the respective enthalpies Delta H-b and entropies Delta S-b together with the specific heat Delta c(p). A strong enthalpy-entropy cancelation was found similar to the results for many other systems. The binding free energy Delta G(b) could furthermore be split up into a part Delta G(ci) due to counterion release and a residual part Delta G(res). The latter quantity reflects specific contributions as, e.g., salt bridges, van der Waals interactions, or hydrogen bonds. The entire analysis shows that heparin-lysozyme interactions are mainly caused by counterion release; that is, ca. three counterions are being released upon binding one lysozyme molecule. Our reported approach of quantifying interactions between glycosaminoglycans and proteins is generally applicable and suitable to provide new insights in the physical modulation of biomolecular signals.