Immunoassays are rapidly replacing many other methods used to detect or quanti-tate substances with important biologic or pharmacologic properties. Enzyme-immunoassays (EIA) are classified into two groups: the heterogeneous EIA, in which the enzyme-labeled antigen or antibody is separated from the enzyme-labeled antigen-antibody complex before measurement of enzyme activity in either fraction, and the homogeneous EIA, in which the enzyme activity of labeled antigen is measured in the presence of labeled antigen-antibody complex, the enzyme moiety of which is sterically inhibited. The enzyme-linked immunosorbent assay (ELISA), a heterogeneous EIA based on the principles as the radioimmunoassay (RIA), has been the subject of several reviews. The only major difference between ELISA and RIA is the use of an enzyme to label the antigen or antibody, rather than a radioactive isotope. A far more serious problem in the application of the competitive ELISA, is the incubation of enzyme-labeled antigens or antibodies with test solutions containing serum, urine, or tissue extract.