The reactivity of the antiserum is directed to the Fc subunit of the IgG molecule. The antiserum contains antibodies to subclass-specific determinants as well as to determinants shared by two or more subclasses of IgG. In immunoelectrophoresis and double radial immunodiffusion, using various antiserum concentrations against normal mouse plasma, serum, and immunoglobulin fractions, no reaction is obtained with IgA, IgM and IgG/Fab fragments or any other serum protein. Direct immunofluorescence staining of cytoplasmic Ig of appropriately treated cell and tissue substrates; to demonstrate immunoglobulins or specific antibodies in cells and tissues; to identify circulating antibodies in serodiagnostic microbiology and autoimmune diseases. 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.