Enzyme-linked immunoadsorbent assay (EIA) has widespread use for the measurement of antibody concentration. The affinity constant (Kaff) of the antibody has an effect upon the quantification by EIA. It is thus important to be able to measure Kaff by solid-phase EIA. Based upon the Law of Mass Action and using serial dilutions of both antigens (coating the plate) and antibody, Kaff has been measured by EIA. A microtiter plate was coated with antigen (Ag) and then incubated with monoclonal antibody (Ab). The plate was sequentially incubated with a second enzyme-antibody conjugate (EAC) and with the enzyme substrate. The amount of Ab adherent to Ag on the plate [Ag Ab] and [Ag2 Ab] was reflected by the enzyme product measured by OD. The use of serial dilutions of Ab resulted in a sigmoid curve of OD versus logarithm of total Ab added to the well. Comparison of the OD at the upper plateau (OD-100) for different antibodies was a reflection of the relative number of epitopes on the Ag that were identified by the different antibodies, provided excessive EAC was used. [Ab]t and [Ab']t were the measurable total antibody concentrations in the wells at OD-50 and OD-50' for plates coated with [Ag] and [Ag'], respectively. [Ag] and [Ag'] were not true antigen concentrations, but were a measure of antigen density on the plate. For [Ag'] = [Ag]/2, Kaff = 1/2(2[Ab']t-[Ab]t. Using five different anti-CEA antibodies and different proportions of CEA in the coating solution, Kaff was measured. Kaff determined by EIA correlated well with Kaff measured by soluble phase inhibition assay. This EIA method of estimation of Kaff is simple, rapid, and reliable.