The distribution of tenascin and fibronectin (plasma fibronectin) was studied immunohistochemically in ameloblastomas, ameloblastic fibromas and ameloblastic carcinomas, as well as in tooth germs using monoclonal antibodies. Tenascin is an extracellular matrix molecule that was shown to be enriched in the embryonic mesenchyme surrounding the budding epithelium in various organs, including the tooth. Tenascin was strongly expressed in the basement membrane zone of the ameloblastomas and in the early tooth germ and the dental lamina, but not in the dental follicle. The expression of tenascin in the ameloblastic fibroma was seen in the basement membrane of the epithelial islands throughout the stromal tissues. There were clear differences in fibronectin expression in the follicular ameloblastoma and ameloblastic carcinoma. The results suggest that tenascin and fibronectin are involved in epithelial mesenchymal interactions of the tooth germ and in odontogenic tumours.