Immunoperoxidase techniques can be easily performed on formalin fixed renal tissue after incubation in trypsin. The sensitivity of the PAP technique on fixed tissue is as good as immunofluorescence methods using frozen sections. The method provides permanent preparations with clear localization of deposited immunoproteins giving accurate classification of renal disease and assessment for treatment. The availability of this technique for retrospective and centralized studies is a major advantage. Infectious agents are rendered harmless and can be specifically identified. The development of the trypsin-immunoperoxidase technique is an important and exciting advance in renal immunopathology.