Immunoassays using enzyme labeled antibodies are highly specific for the analysis of a particular protein. Use of an enzyme labeled antibody together with a highly sensitive TMB substrate provides an excellent method for the detection and characterization of sample proteins bound to solid surface by ELISA techniques. Following attachment of protein to the solid surface (ELISA plate), a primary antibody is used to selectively bind the protein of interest. Alternatively, a known protein is bound to the surface for the screening of specific monoclonal/polyclonal antibodies in serum or other samples. An enzyme labeled second antibody directed against the primary antibody is then applied. The TMB substrate reacts with bound HRP to produce blue color. Upon addition of stop solution, the color is converted to yellow color (measure at 450nm). The yellow color is more intense and thus more sensitive than the blue color. Intensity of color, up to a certain range, is directly proportional to the amount of bound primary antibody.