Cell culture techniques were used to clarify the histogenesis of giant-cell tumor of bone. Even after passage, nearly all of the mononuclear cells possessed tartrate-resistant acid phosphatase and receptors for eel calcitonin, which are both phenotypic markers for osteoclasts. Eel calcitonin produced an increase in the cyclic adenosine monophosphate (cAMP) content of the mononuclear cells. More than 90% of mononuclear tumor cells expressed monocyte markers; flow cytometric C3b receptor, a macrophage marker, was also detected in a few cells. These findings demonstrate that the mononuclear cells expressed phenotypes of both the osteoclast and monocyte-macrophage and that they originate in a monocyte-macrophage-osteoclast lineage. Giant-cell tumor of bone may thus provide a good model for investigating the mechanism of bone resorption in which cells of osteoclast lineage play a central role.