Established histochemical procedures have been altered by utilizing reagents which contain a group capable of reacting selectively with OsO(4). This permits the cytochemical demonstration of a particular enzyme or functional group of a macromolecule with the light and electron microscope. The osmiophilic groups used thus far are mercapto, thiocarbamyl, and diazothioether. The "osmium black" produced at the site of reaction is electron opaque, is insoluble in lipids, and is not altered when the issue is embedded in acrylic and epoxy resins. Its fine amorphous character enables it to be maintained precisely in the electron beam.