The reactivity of the antiserum is directed to the Fc subunit of the IgE molecule, which expresses strict (class) specificity. In immunoelectrophoresis and radial in immunodiffusion using various antiserum concentrations against serum of mice belonging to different allotypic groups, a single precipitin line has been obtained which shows a reaction of identity with the precipitin lines obtained with the purified IgE used as immunogens. It does not react with IgG including all subclasses, IgG/Fab fragments, IgM and IgA or any non-Ig protein in mouse serum, as tested by immunoelectrophoresis and double radial immunodiffusion. The antiserum also react with membrane-bound IgE in peripheral blood cells of different mouse strains as tested by immunofluorescence microscopy. In immunocytochemical and immunohistochemical staining for the detection of IgE at the cellular and subcellular level by staining of appropriately treated cell and tissue substrates; to identify and measure IgE in mouse serum or other body fluids. This immunoconjugate is not pre-diluted. The optimum working dilution of each conjugate should be established by titration before being used. Excess labelled antibody must be avoided because it may cause high unspecific background staining and interfere with the specific signal.