Histofine Simple Stain MAX Peroxidase (PO) is a detection reagent designed specifically for immunohistochemical staining on formalin-fixed paraffin-embedded rat tissue sections. This reagent incorporates the Universal Immuno-Enzyme Polymer Technology developed by Nichirei Biochemicals to create a labeled polymer prepared by combining amino acid polymers with peroxidase and secondary detection agent, which is reduced to Fab' fragment (not a whole antibody). To further eliminate background staining, solid-phase absorption of the secondary antibody was carried out with rat, human, dog, pig, and bovine sera.