PAP soluble immune complexes are prepared by the method of Sternberger et al. (J. Histochem. Cytochem. 1970. 18, 315). Theoretically, they consist predominantly of two anti-horseradish peroxidase antibodies in soluble complex with three molecules of horseradish peroxidase. They are economical reagents for sensitive (Sternberger and Sternberger, J. Histochem. Cytochem. 1986. 34, 599) immunological detection of primary antibodies. Although higher sensitivity may be obtained with newer reagents such as horseradish peroxidase-conjugated streptavidin (J. Histochem. Cytochem. 1988. 36, 317; and J. Histochem. Cytochem. 1989. 37, 1609), PAP continues to be used by some investigators, apparently due to its low non-specific binding, which results in little or no background staining. F(ab')₂ PAP has been used to prevent the binding of PAP to Fc receptors.

PAP soluble complexes are normally used at 25-50 µg/ml.  Whole antisera against IgG (H+L) are recommended for bridging PAP to primary antibodies. Antisera against IgG (H+L) or against the F(ab')₂ fragments of IgG will also bridge PAP to IgM primary antibodies (such as IgM monoclonal antibodies) by virtue of common light-chain recognition.  Normal serums from the same host species as the bridging antibodies are suggested as blocking agents to minimize non-specific binding. All PAP soluble complexes are freeze dried at high concentrations for stability and high-dilution capability.

PAP soluble complexes, as well as other immunoperoxidase reagents, are not recommended for tissues or cells in which endogenous peroxidase activity is difficult to suppress. In such cases, other immunoenzyme reagents may be used. Alternatively, anti-horseradish peroxidase conjugated with other enzymes or fluorophores may be used to enhance signal and reduce background since the final signal does not depend on the enzyme activity of peroxidase but on the antigenicity of horseradish peroxidase.