The values of apparent adiabatic compressibility of free and antigen-bound antibodies were determined by means of precise density and ultrasound velocity measurements. It was shown that during the formation of soluble immune complexes (insulin--monoclonal antibodies to insulin and alpha-amylase--monovalent Fab-fragments of antibodies to alpha-amylase), the apparent compressibility of antibodies decreased by (0.3 divided by 0.9).10(-6) cm3/g.bar. During the formation of large insoluble aggregates (alpha-amylase--polyclonal antibodies to alpha-amylase), the apparent compressibility decreased by (5.5 +/- 0.7).10(-6) cm3/g.bar. It is suggested that the decrease in the magnitude of thermal fluctuations of the molecular volume of antibodies during antigen binding, manifesting itself by the decrease in their compressibility and strengthened several-fold by precipitate formation, may favour the activation of the effectory functions of antibodies.