A rapid ELISA method for measuring association constants of monoclonal antibodies against protein antigens is proposed. Two microtiter plates were used; one plate was used for equilibrating monoclonal antibody solutions of various concentrations with antigens in wells of the plate and the other for measuring the equilibrated concentrations of the antibody by ELISA. It was possible to determine the association constants within a few hours by this method and apply the method to association constants larger than those measured by the equilibrium dialysis method.