Although affinity and kinetic measurements on their own provide useful information regarding the suitability of antibodies for various immunoassays, a thermodynamic analysis provices additional information that throws light on the molecular forces at work in the antigen-antibody interaction. It may then be possible to adjust assay conditions in order to favour either the association or dissociation of antigen-antibody complexes. A tenfold increase in binding affinity (K) corresponds to a free energy change of only 1.4 kcal/mol (5.8 kJ/mol) at 25 degrees C. This means that K values of 10(5) M-1 and 10(10) M-1 correspond to a free energy change (delta G) of 7.0 and 14.0 kcal/mol respectively. The entire range of affinity constants normally encountered in antigen-antibody interactions, therefore differs by no more than about 7 kcal/mol of free energy change, which is equivalent to only a few hydrogen bonds. In comparison, a single electrostatic interaction corresponds to about 4 kcal/mol of free energy change. A full description of the binding interaction requires an understanding of the change in hydration states of the reactants when the complex forms, and an assessment of the entropic andenthalpic effects of these changes. Contrary to earlier assumptions, it is now clear that antigen-antibody interactions are often accompanied by a large favourable enthalpy which more than compensates the unfavourable entropy.