Rabbit anti-dinitrophenyl antibody from a serum pool was obtained as five fractions of purified specific antibody by a limiting antigen precipitation method. Each fraction had a different binding affinity for epsilon-N-2,4-dinitrophenyl-L-lysine. The free energy changes for hapten binding to the five antibody fractions varied from -8.35 to -10.0 kcal/mol. An average deltaH of -13.9 kcal/mol was measured for the fractions with a batch calorimeter. The results indicate no significant correlation between enthalpy changes and free energy changes. However, a statistically significant correlation between the free energy changes and the entropy changes was found. The enthalpy of the anti-dinitrophenyl antibody interaction with multivalent dinitrophenyl human serum albumin was determined. These are the first enthalpy measurements of an antibody antigen reaction in which the intrinsic binding enthalpy between the antibody and the determinant group is known. The deltaH for the antigen binding reaction was -10.1 kcal/mol which is 3.8 kcal/mol less exothermic than the deltaH for the hapten binding reaction. The interactions that could lead to such a difference in enthalpy are discussed.