The heterogeneous enzyme-immunoassays has at least one separation step which allows the differentiation of reacted from unreacted material. An essential difference from homogeneous enzyme-immunoassays is that in heterogeneous assays the enzyme on the labeled antigen or antibody retains its activity even after its reaction with the reciprocal antibody or antigen. The enzyme to be used as a label must satisfy the following criteria: price, stability, capability of being linked to antibody or antigen, and have a suitable chromogenic substrate. Either antigens or antibodies may be labeled with the enzyme. In virtually all of the described enzyme-immunoassays either the antigen or antibody is immobilized onto a solid phase. Reproducibility is best achieved by the inclusion of reference samples both in the production stage of reagents and in the actual Enzyme-linked immunosorbent assay (ELISA) tests on a day-to-day basis. These reference materials are available for many hormones and other haptens.