Several studies have suggested that the osteoclast is derived from a mononuclear precursor which is found in bone marrow. We have developed a system for studying the formation of osteoclast‐like multinucleated cells in long‐term bone marrow culture of baboon cells. Recombinant human CSF‐GM and highly purified CSF‐1, both of which stimulate the proliferation of monocyte/macrophage precursors, were found to increase the number of osteoclast‐like cells formed in long‐term bone marrow culture. CSF‐GM stimulated multinucleated cell formation more consistently than CSF‐1. The subsequent addition of 1,25‐dihydroxyvitamin D₃ (1,25‐(OH)₂D₃) to cultures initially treated with CSF‐GM or CSF‐1 further increased multinucleated cell formation. Autoradiographic studies indicate that CSF stimulated multinucleated cell formation by increasing the proliferation of the precursor cell, and that the potentiating effect of 1,25‐(OH)₂D₃ was caused by fusion of the increased numbers of precursors. These studies suggest that the interaction of locally produced colony‐stimulating factors with circulating calcium regulating hormones may be important in the control of osteoclast formation and bone resorption.