Adamantinoma of the tibia was studied by electron microscopy and immunohistochemistry. The palisading tumor cell nests were invested with a continuous basal lamina. The peripheral tumor cells of the nests were bound to each other by desmosomes. Intermediate densities in the form of incomplete hemidesmosomes were seen in the outermost cells of the nests. There were two types of cytoplasmic filaments--tonofilaments and actinlike microfilaments. The latter occurred predominantly in the cytoplasm beneath the plasma membrane of the outermost cells of the nests. This could be implicated in the motile and invasive properties of the tumor cells. Immunohistochemic stains localized keratin in most tumor cells. Coagulation factor VIII-related antigen was localized in the endothelial cells of the stroma but not in the tumor cells. These findings indicate the appendicular adamantinoma to be of ectodermal epithelial origin, most likely a variant of basal cell carcinoma.