An immunoperoxidase method was used to detect involucrin in 47 odontogenic tumors and 35 radicular cysts. Of a total of 40 ameloblastomas, 9 cases were positive for involucrin expression and those positive cases exhibited acanthomatous or follicular patterns. Squamous odontogenic tumors were strongly positive for involucrin, whereas adenomatoid odontogenic tumors gave a negative staining reaction. Involucrin expression in odontogenic tumors was divided into three categories: single cell positive, focally positive, and squamous metaplastic cell positive. Radicular cysts showed a very irregular distribution of involucrin; nonstratified epithelium was generally negative or showed only trace staining for involucrin, whereas suprabasilar stratified squamous epithelial cells were strongly positive. Cells positive for involucrin in odontogenic tumors and in cystic epithelium are probably direct signs of epithelial differentiation; such cells were squamoid in appearance.