The identification and purification of human osteoclast precursors is essential to further our understanding of the mechanisms that control human osteoclast differentiation. Osteoclastoma tissue potentially provides a rich source of human osteoclast precursors, and in previous studies we have demonstrated the existence of a population of mononuclear cells within this tissue that is reactive with osteoclast-selective vitronectin receptor monoclonal antibodies. In this study, mononuclear cells expressing the vitronectin receptor, as defined by their ability to react with a murine monoclonal antibody to the beta 3 chain of the vitronectin receptor (87MEM1), were isolated from collagenase digests of osteoclastoma tissue using a fluorescence activated cell sorter. Based on their fluorescence signal and size, approximately 2-3% of the viable cells (typically 2 x 10(5)) were obtained and prepared for further phenotyping. The isolated cells demonstrated a number of phenotypic characteristics of osteoclasts: positive tartrate-resistant acid phosphatase (TRAP) activity, reactivity with human osteoclast-selective antibodies, expression of calcitonin receptors, cathepsin K (a novel osteoclast-selective cysteine proteinase) mRNA, and osteopontin mRNA and protein. These phenotypic characteristics were also detected in mononuclear cells within cryostat sections of the native osteoclastoma tissue as well as in resorption lacunae of sections of human bone. In contrast, isolated peripheral blood monocytes were negative for TRAP activity and osteopontin expression and, unlike the osteoclastoma-derived cells, demonstrated strong nonspecific esterase activity. Significantly, when the osteoclastoma-derived 87MEM1 positive cells were cocultured on whale dentine for 1-3 weeks with stromal cells, extensive resorption of the dentine surface was observed. This is the first demonstration of the purification of human osteoclast precursors. These cells provide an homogeneous cell population for studying cellular events that occur during human osteoclast differentiation.