Pretreatment of epoxy thin sections with strong oxidizing agents such as hydrogen peroxide, sodium methoxide, and sodium m-metaperiodate facilities the location of antigens with immunostaining procedures. Etching, or pretreatment, of sections unmasks antigenic sites on glutaraldehyde fixed and postosmicated tissue, partially removes osmium bonds, temporarily decreases the hydrophobicity of the epoxy surface layer of the section, reduces the electron density of the tissue and increases resistance to heavy rnetai poststaining.