The nature of the tumor cells in 5 eases of ameloblastomas was studied by immunohistochemistry, and the findings were compared with developing mouse and human teeth as well as with 5 cases of carcinomas in the oral region. The antigens investigated were keratin, an intracellular cytoskeletal protein typical of epithelial cells, and laminin, an extracellular matrix protein found in basement membranes. Our results show that keratin is expressed by all types of epithelial cells in ameloblastomas as well as in the epidermoid carcinomas, and developing teeth. The epithelial, keratin‐positive tumor islands in the ameloblastomas were surrounded by a continuous line of laminin, in a pattern similar to that seen in developing tooth. Laminin was seen also around the epidermoid carcinomas but large areas devoid of laminin were constantly seen between the stroma and the neoplastic epithelium. This indicates a lack of proper basement membrane formation by the malignant epidermoid carcinomas. This may be due either to a diminished production or an increased degradation of basement membrane proteins by the carcinoma cells. Our results are in line with suggestions that ameloblastomas are derived from odontogenic epithelial cells. Immunostaining for keratin does not distinguish between carcinomas and the ameloblastomas. However, visualization of basement membrane proteins such as laminin can apparently be used in the differential diagnosis between ameloblastomas and carcinoma