Alkaline phosphatase activity was demonstrated in the granular cell layer of cholesteatoma epithelium, ear canal and postauricularin skin by blocking the cysteine-containing active sulfhydryl groups with iodoacetic acid. Alkaline phosphatase activity occurred only at pH 9.4. Substitution of lecithin for glycerophosphate did not result in demonstration of enzyme activity. Since alkaline phosphatase activity is very strong in columnar epithelium, it is speculated that inhibition of its activity may be required for the formation of keratinizing epithelium.