The reactivity of the antiserum is directed to the subclass IgG2b as tested against rat sera and a panel of purified homogenous immunoglobulins. It does not react with other subclasses of IgG, IgG/Fab fragments, IgM and IgA or any non-Ig protein in rat serum, as tested by immunoelectrophoresis and double radial immunodiffusion. Further tested on the level of sensitivity of direct and indirect immunofluorescence staining methods applied to lymphoid cells. When used on vital peripheral blood cells this immunoconjugate does not react with Fc receptors present on cell surface membranes. To identify and measure IgG2b, antigen or antibody, at the cellular and subcellular level by immunofluorescence staining of appropriately treated cell and tissue substrates, and to demonstrate circulating antibodies in serodiagnostic microbiology and autoimmune diseases; to identify a specific antigen or immune complex using a reference antibody of rat origin in the middle layer of the indirect test procedure. 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.