The light-microscopic and ultrastructural findings in a case of so-called giant cell tumor of soft parts, localized at the dorsal side of the left foot of a 23-years-old male are described. An amputation of the lower extremity was performed and subsequently chemotherapy with adriamycin was given for 3 months. Despite the histology and cytologic malignant appearance and the evident vascular invasion, already present at the time of the first excision, the last known status 2 years later seems favorable. There are no pathologic lymph nodes in the groins and no signs of metastases on chest X-rays. From the electron-microscope study no definite conclusion can be drawn as regards the histogenesis of this tumor. we feel, as do others, that many of the principal mononuclear tumor cells are poorly differentiated mesenchymal cells. Some of the neoplastic cells, however, show ultrastructural features suggestive of chondro- or osteoblasts (a well-developed r.e.r. containing electron-dense material; multiple Golgi complexes; masses of glycogen; interdigitating cell membrane villi; cytoplasmic filaments; an extracellular amorphous matrix). Some of the larger tumor cells have the submicroscopic aspects of histiocytes as described in osseous, cutaneous, or pulmonary lesions of the histiocytosis X group. As yet undetermined cytoplasmic inclusion bodies constitute another rare observation in our material.