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.

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 serum (5% v/v) from the same host species as the bridging antibodies is recommended as a blocking solution to minimize non-specific binding. All PAP soluble complexes are freeze-dried at 20 mg/ml.

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 (HRP) antibodies conjugated with alkaline phosphatase or fluorophores, or complexed with colloidal gold particles may be used to enhance signal and reduce background from endogenous enzymes, since the final signal does not depend on the enzyme activity of peroxidase, but on the antigenicity of horseradish peroxidase.