The purpose of this study was to design a method by which immunoperoxidase staining can be applied to formalin-fixed, paraffin-embedded tissue sections to demonstrate amyloid deposits in cerebral and systemic amyloidotic tissues. We used anti-prion protein, anti-beta-protein, anti-amyloid A, and anti-prealbumin antisera. The tissue sections were first treated with 100% formic acid for 5, 20, or 60 minutes and the unlabeled immunoperoxidase method (biotin-streptavidin system reagents) was used. This formic acid pretreatment enhanced immunoreactivity of the amyloid deposits which reacted positively with specific antiserum. The specificity of the immunostainings was well preserved. This method can also be used to demonstrate interspecies cross-reactivity, by using anti-human amyloid A and anti-scrapie hamster prion protein antisera, which stained negatively or faintly with amyloid deposits of heterogenous species. The technique is expected to reveal the buried epitopes of amyloid deposits in tissue sections.