SYNOPSISA procedure for the application of cytochemical techniques directly on ultrathin sections for electron microscopy has been developed with the use of water‐soluble plastics, which act both as dehydrating agents and embedding media. Our approach has been to selectively extract proteins and nucleic acids from ultrathin sections of cells and viruses by specific enzyme hydrolysis. Glycol methacrylate is particularly useful as a component of the embedding medium because it enhances the penetrability of the enzyme into the section and preserves the biological materials in such a way that they remain susceptible to enzymic digestion. Various degrees of morphological preservation of ultrastructure are obtained with the aldehyde fixatives, formalin, acrplein, and glutaraldehyde, followed by. embedding in glycol methacrylate and the choice of fixative, at present, depends on the system which is to be studied. It is believed that the proposed method can be developed further and be used for other approaches to ultrastructural cytochemistr