Ultrastructural examination of 26 osteogenic sarcomas has revealed that in addition to the basic osteoblast comprising the tumor, five additional cell types may be identified. These include chondroblasts, osteocytes, undifferentiated cells and myofibroblasts. The latter cell type has not been reported in previous studies of medullary osteosarcomas. The relative portions of the cell types varied as to the type of osteogenic sarcoma studied. Osteoid production, the single diagnostic feature of these tumors, was evident and the various ultrastructural appearances of this material are shown. One case of an osteogenic sarcoma that arose in Paget's disease of bone is described and did not differ ultrastructurally from the conventional medullary osteogenic sarcoma. Our studies have shown that osteogenic sarcoma may share common ultrastructural features with other sarcomas involving bone and soft tissue and that the basic difference between osteogenic sarcoma and soft-tissue sarcomas is the presence of osteoid in the former and differing cytoplasmic differentiation in the latter category of tumors.