The reactivity of the antiserum is restricted to a subclass specific determinant on the Fc part of the IgG1 molecule as tested in direct binding enzyme immunoassay, immunoblotting, immunoprecipitation and direct immunoperoxidase staining of cytoplasmic Ig. To identify the presence of IgG1 in human serum, other body fluids, cell and tissue substrates and to determine its concentration in techniques as ELISA, indirect immunoperoxidase staining of cytoplasmic IgG1, and immunoblotting using a peroxidase labelled monoclonal antibody against TRITC. The optimum working dilution is an assay-related characteristic. It may vary widely and should always be determined by titration. For histochemical use optimum dilutions are mostly from 1:10 to 1:50; in ELISA from 1:200 upwards; in Western blotting from 1:400 upwards.