Comparative ultrastructural and histoenzymological study of 10 osteogenic (4 osteosarcomas), osteolytic (3 benign giant cell tumours, 2 malignant osteoclastomas) and fibrosarcomatous osseous tumours of the jaw emphasised the analogies between osteoblasts and fibroblastic stromal cells of giant cell tumours. These two cell types, with their abundant granular ergastoplasm, secrete a collagen interstitial fibrillary substance which is capable of secondary calcification. Their enzyme activities, with regard to oxidative mechanisms and protein synthesis are identical. Osteogenic activity as assessed by ATPase and alkaline phosphatase, more marked than in osteosarcomas, is also found in the stroma of benign giant cell tumours. With regard to the plasmodes characteristic of giant cell tumours, they are certainly original from both a morphological and enzymological standpoint : richness in mitochondria and lysosomes, intensity of oxidative reactions (B.O.H.B.D. +++). Nevertheless, the morphological appearances seen would be more in favour of their stromal origin than in their arising from monocytes-macrophages coming from the blood.