The reactivity of the antiserum is restricted to a isotype specific determinant of the IgE molecule as tested in indirect binding enzyme immunoassay, immunoblotting, immunoprecipitation and direct immunofluorescence staining of cytoplasmic Ig. To identify the presence of IgE in human serum, other body fluids, cell and tissue substrates and to determine its concentration in techniques as direct and indirect immunofluorescence staining of cytoplasmic IgE and cell membrane IgE. The optimum working dilution is an assay-related characteristic and should always be determined by titration. For histochemical use optimum dilutions are mostly from 1:50 to 1:200. These data should be interpreted as general recommendations only.