The ameloblastoma is usually thought to be devoid of any odontogenic capacity. However, the histological, ultrastructural and histoenzymological study of 5 cases demonstrates the fairly high level of differentiation of some ameloblastic cells in such neoplasms. Furthermore, it suggests very early stage of epithelio-conjunctive interaction. The tumours consist, in addition to stellate, squamous, clear and dark epithelial cells, of densely packed peripheral columnar cells similar to the differentiated ameloblasts of the inner epithelium of enamel organ (median nucleus, well-developed rough-surfaced endoplasmic reticulum, mitochondria and lysosomal-vacuolar system located in the distal end of the cells). Besides, the cytoplasmic membrane shows some pocket-like apical invaginations similar to early Tomes processes. On the other hand, similarly to physiological odontogenesis, an abundant amount of granulo-filamentous material is found along the lateral cellular membranes, and the basement membrane. This substance, sometimes, is intermingled with vertical collagenous fibers. In some areas, it is disrupted and cytoplasmic invaginations of epithelial cells, passing through these gaps, are connected with neighbouring mesenchymatous cells. In other respects, despite the lack of any odontoblastic differentiation, the high level of alkaline phosphatase activity found in these cells by means of histo and cytoenzymological technics, looks like that of odontogenic mesenchymal cells. These findings seem to demonstrate the histogenetic unicity of odontogenic tumours. These neoplasms, like the embryonary dental organ, are provided with two types of cells (epithelial and stromal cells). Thus, they are able to induce odontogenesis. But, in ameloblastoma, this power of induction is restricted to a very early stage of odontogenesis.