The azo dyes were found to react with antibodies aggregated in immune complex in a similar way to heat-aggregated IgG. The whole micelles of the dye, instead of single molecules, are fixed to antibodies. In a consequence, the number of dye molecules, determined per one antibody molecule, differs but may be as large as 50-60. The dye, bound to antibody complexed with antigen, enhances its affinity to antigen. The increase of affinity, expressed as the relation of changed by the dye association constants to its initial value, was found to be 6.27 x 10(8):1. The corresponding energy change, calculated from this value, equals 11.97 kcal/mol. The resulting amount of antibodies, bound to antigen, is essentially enlarged. The enhancement effect does not seem to depend on the number of antibodies bound to the red cell up to about 3 x 10(4) molecules. It indicates that in this range a single antibody binding may be affected by the dye. The situation changes at growing antibody densities in the complex when the dye-antibody network, formed by crossbridged molecules, additionally increases the stability of the immunoglobulin molecules engaged in the complex.