The peroxidase anti-peroxidse (PAP) method of staining is useful in light and electron microscopic immuno-cytochemistry. Utilizing three reagents- the primary antibody, the secondary bridging antibody, and the peroxidase anti-peroxidase complex (PAP)- a highly efficient system is formed. The PAP complex contains three molecules of enzyme and therefore is highly sensitive for its relatively low molecular weight. Each PAP reagent includes a product data sheet with all pertinent information and instructions.