Electron microscopic study of 9 osteosarcomas corroborates the light microscopic observation that cells with osteoblastic, chondroblastic and fibroblastic features occur in osteosarcomas. Two types of multinucleated giant cells were observed, one was osteoclast-like. The other had anaplastic and degenerative changes. The intercellular matrix is composed of collagen fibers, amorphous ground substance with osteoid and tumor bone. Contrary to many previous electron microscopic reports, osteosarcomas contain more than one cell type.