Primary malignant bone tumors, osteosarcomas (9 cases), and Ewing's sarcomas (10 cases) were examined for their reactivities with monoclonal and polyclonal antibodies against filamentous proteins and cell membrane determinants of the lymphoid and macrophage marker series. The reactivity of antibodies was studied on snap-frozen tissue probes by using a triple layer immunoperoxidase method. Osteosarcomas were positive for vimentin and, in part, for HLA-DR. Other types of intermediate-sized filaments were not detected in tumour cells. In a small number of cases (2/9) tumour cells were reactive with antibodies of the macrophage series (Leu M2). In Ewing's sarcomas, vimentin and HLA-DR was also demonstrated. It was particularly interesting that Leu M2 staining was found in the majority of cases (8/10). The staining pattern supports the assumption that this peculiar tumour is of mesenchymal (monocyte/macrophage) histogenesis. It was evident from the present study that, in primary osteogenic tumors, none of the examined tumour "markers" were as distinctive as they are for bone metastases. Nevertheless, the reactivity of Ewing's sarcoma cells with monoclonal antibodies of the Leu M2 type throws some highlights on the, as yet, obscure histogenesis of the neoplasm and may be of diagnostic value in conjunction with the known light and electron microscope features of the tumour.
