We have presented our detailed methods for localizing antigens in cultured cells and tissue sections by IP at the EM level. Immunoperoxidase cytochemistry is particularly well suited for the study of sparse antigens as a result of the enzymatic amplification afforded by the method, and of molecules confined within a membrane-enclosed compartment wherein the DAB reaction produce can accumulate. Although IP is commonly used to localize membrane-compartmentalized molecules, reliable qualitative information can also be obtained on cytoplasmic antigens as well (Anderson et al., 1978; Merisko et al., 1986; Rodman et al., 1984). For these and other reasons, it is likely that IP cytochemistry will continue to be an important tool for the cell biologist especially in the study of membrane traffic. Other inventive combinations of immunocytochemical methods will likely be forthcoming, for example, combining IP localization with postembedding labeling by colloidal-gold conjugates to provide triple EM labeling.