Free energy of antigen-antibody binding has been calculated for HyHEL-5, HyHEL-10, and D1.3 complexes. We also have calculated free energies of binding per residue of L- and H- chains of the antibodies, and those of the antigen (lysozyme). The results of the calculations provide support for the notion that TYR and TRP residues may confer on the CDRs of antibodies an enhanced capacity for binding antigens. It was shown also that the composition of residues that provide major part of the binding free energy differs for antibodies and antigens.