The ultrastructural examination of four osteosarcomas (osteogenic, undifferentiated, and pleomorphic) is described. There are three types of tumor cells. Most of the cells are held in contact by desmosome-like tight junctions; they are atypical osteoblasts with cytoplasmic processes, dilated rough endoplasmic reticulum, mitochondria carrying calcific inclusions, lipid droplets surrounded by glycogen, and intracellular fine filamentous fibers. Other cells exhibiting varying degrees of osteoblastic maturity are also seen with multilobed nuclei, a clear cytoplasm, and straight bordered membranes. The last type is chondroid with abundant deposits of glycogen, lipid droplets, and undilated rough endoplasmic reticulum. The matrix is composed of fibrils without periodicity, scattered and deteriorated collagen fibers, and focal calcium deposits of hydroxyapatite crystals as in embryonal bone, dentine, or callus bone.