Spindle-shaped cells were established from four giant-cell tumors of bone. When human blood monocytes were co-cultured with these cells, multinucleated giant-cell formation of monocytes was induced. Intriguingly, even when a filter (pore size: 0.45 microm) was interposed between monocytes and the spindle-shaped cells, polykaryocytes also appeared. These multinucleated giant cells were positive for tartrate-resistant acid phosphatase, expressed calcitonin receptor, and showed bone-resorption activity, characteristics of osteoclast-like cells. These findings indicate that soluble factors secreted from these cells play an important role in osteoclast-like cell formation from blood monocytes. These data additionally suggest that these cells support osteoclast-like cell formation in giant-cell tumors of bone. The cells also expressed mannose receptor, fibronectin, receptor activator of nuclear factorkappaB, and several cytokine mRNAs, including interleukin-6, receptor activator of nuclear factorkappaB ligand/osteoclast differentiation factor/osteoprotegerin ligand, and macrophage colony-stimulating factor. However, all of these molecules except receptor activator of nuclear factorkappaB ligand mRNA could also be detected in control HeLa and CV-1 cells. Although the soluble receptor activator of nuclear factorkappaB ligand has not been found under physiological conditions, it is possible that it is cleaved by cellular proteases and the truncated receptor activator of nuclear factorkappaB is released from cells. Identification of the soluble factors capable of inducing osteoclast formation from blood monocytes is a pressing problem to be solved.