Acid phosphatase activity has been studied in normal and X‐irradiated rat epidermis. A remarkable reaction increment was found in irradiated epithelium at the granular layer keeping the same pattern of distribution observed in the normal tissue. No reactive lysosomes or keratinosomes were observed in the cytochemical sections, while many keratinosomes but no lysosomes appeared in the electron microscopic preparations of irradiated animals. Irradiated epithelium is discussed as a suitable model to analyze the role of acid phosphatase in the keratinization mechanism