Circulating CD34+ cells were isolated from leukapheresis products collected from patients with ovarian cancer. CD34 contaminating cells, identified immediately after immunoselection, ranged from 5% to 25% in five different experiments and were predominantly CD3+ T-lymphocytes (range 2-12%), CD3+/CD16+/CD56+ natural killer cells (range 2-11%) and rare mature CD15+/ CD11b+ granulocytes (range 1-2%). CD34+ cells were cultured in liquid medium in the presence of interleukin-3, granulocyte-macrophage colony stimulating factor. stem cell factor, granulocyte colony stimulating factor and a powerful proliferation with prevalent differentiation along the granulocytic/monocytic lineage was obtained. After 10 d of culture a small but consistent number of early multinucleated osteoclasts were identified with a frequency of one cell per 700 granulocytic/monocytic cells, as revealed by cytologic examination. This observation was confirmed by staining for tartrate-resistant acid phosphatase activity which revealed red multinucleated elements with a frequency comparable to that reported above. Conversely, no osteoclasts were observed in those cultures in which macrophage overgrowth was obtained by culturing CD34+ cells until day 35. These observations suggest that circulating progenitors have a multilineage potential in vitro and contribute to the clarification of osteoclast development in humans: additionally, they provide the basis for the future development of optimized osteoclast culture techniques in liquid medium and the basic culture system, to test the distinct activity of 1,25(OH)2D3. parathyroid hormone interleukin-11 and of other cytokines on osteoclast development in humans.