The reactivity of the antiserum is directed to the Fc subunits of the major isotypes of the monkey immunoglobulin system. No reaction is obtained with purified IgG/Fab or any non-Ig protein of monkey serum, as tested by immunoelectrophoresis and double radial immunodiffusion. In immunoelectrophoresis and double radial immunodiffusion using various antiserum concentrations against normal monkey plasma and serum, the characteristic IgG, IgA and IgM precipitin lines are obtained. In enzyme-immunocytochemical and immunohistochemical staining for the detection of cytoplasmic Ig at the cellular and subcellular level by staining of appropriately treated cell and tissue substrates, and to demonstrate circulating antibodies in serodiagnostic microbiology and autoimmune diseases. The absence of activity to the common Ig/Fab subunit prevents the reaction of this conjugate with immunoglobulins bounds to Fc receptors on non-lymphoid cells. 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.