The in vitro growth pattern of cells obtained from bioptic material of ten patients with giant cell tumor of bone (GCT) was investigated. Cytochemical reactions and monoclonal antibodies raised against macrophage markers were tested on the two histologically identifiable GCT cell populations. Only monoclonal antibody EBM/11 stained both mononuclear and giant cells. EBM/11 positivity and resistance of acid phosphatase to high doses of tartrate strongly suggest that both mononuclear and giant cells belong to the same lineage.