In general, immunological methods are not very well suited for a quantitative determination of the antigen to be studied. The ELISA technique, however, can be used for a quantitative or at least semiquantitative determination of the concentration of a certain antigen. The method was first introduced by Engvall and Perlmann (1). The principle of ELISA (see Fig. 1), also called the double antibody sandwich technique, is the following: Antibodies against the antigen to be measured are adsorbed to a solid support, in most cases a polystyrene microtiter plate. After coating the support with antibody and washing, the antigen is added and will bind to the adsorbed antibodies. Next, a conjugate that will also bind to the antigen is added. Conjugates are antibody molecules to which an enzyme is covalently bound. Fig. 1. The main steps in a (noncompetitive) ELISA test. (1) The antibody to the antigen being quantitated is adsorbed onto a solid phase, usually polystyrene. (2) The sample containing the antigen being measured is then added. (3) Following the incubation and washing steps, a second enzyme-labeled antibody is then added. After further incubation and washing steps, enzyme substrate is added. (A substrate is chosen that will give a colored product). The amount of color produced is therefore proportional to the amount of antigen bound to the original antibody.