Immunohistochemical study of giant cell tumor of bone (GCT) was carried out utilizing a panel of monoclonal antibodies including those against lysozyme, alpha-antichymotrypsin (AACT), vimentin (Vim), M718, HLA-DR, S-100 protein, epithelial membrane antigen (EMA), leukocyte common antigen (LCA), KB90, factor VIII related antigen (F VIII) against stromal cells and giant cells in 20 cases of GCT, with one case of prolonged continuously cultured cells GCT (GT 15). The results showed that stromal cells could be divided into two subgroups. Mesenchymal stromal cells labelled only with vimentin and were regarded as being derived from undifferentiated mesenchymal cells of bone marrow, while macrophage-like stromal cells labelled with antigens and were found to be present in mononuclear phagocytes. We believe this to be the first report that some stromal cells reacted positively with S-100 protein. Multinucleated giant cells were AACT and M718 positive, indicating their close relationship to macrophage-like stromal cells. The prolonged cultured cells accepted labelling with vimentin only indicating that all macrophage-like stromal cells disappeared after several subcultures and the only cells that could continue to be subcultured were the mesenchymal stromal cells.