The reactivity of the antiserum is directed to the Fc subunit of the IgA molecule, which expresses strict (class) specificity. In immunoelectrophoresis and radial in immunodiffusion using various antiserum concentrations against chicken serum, a single precipitin line has been obtained which shows a reaction of identity with the precipitin lines obtained with the purified IgA used as immunogens. It does not react with IgG, IgG/Fab fragments and IgM or any non-Ig protein in chicken serum, as tested by immunoelectrophoresis and double radial immunodiffusion. In immunocytochemical and immunohistochemical staining of IgA at the cellular and subcellular level of appropriately treated cell and tissue substrates; to demonstrate circulating IgA antibodies in serodiagnostic microbiology and autoimmune diseases; to identify a specific antigen using a reference antibody of chicken origin known to be of the IgA isotype 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.