Enzymatic activity and cell membrane proteins were characterized in cells from five giant cell tumors of bone (GCTs). Napthyl alpha esterase (NAE) and acid phosphatase (AP) activity was noted within both the mononuclear and multinucleated cells of each tumor. In each tumor, all mononucleated cell populations displayed tartrate-sensitive AP activity, whereas the multinucleated cell populations demonstrated variable expression of tartrate- sensitive and tartrate-resistant AP activity. Analysis of cell membrane proteins included attempts at immunodetection of mannose receptor, OKM-1 antigen (OKM-1a), colony-stimulating factor-1 receptor (CSF-1r), and platelet-derived growth factor receptor (PDGFr). None of these membrane antigens were elicited on multinucleated cells. In contrast, the mannose receptor, OKM-1a, and PDGFr all were detected on the mononucleated cells within each tumor. These data demonstrate that a population of mononucleated, not multinucleated cells, expresses features unique to mature mononuclear phagocytes and establishes the presence of a membrane receptor, PDGFr, associated with mitogenesis of mesenchymal cells.