Osteoclastoma-derived giant cells were used to produce 11 mouse monoclonal antibodies (MAb) reactive against human osteoclasts on undecalcified sections of adult human bone. All exhibited unique reactivities across a wide range of human tissues. Three in particular demonstrated distinctive reactivities; C35 was highly selective for bone osteoclasts, C27 showed selective reactivity for osteoclasts, tissue macrophages and blood-borne monocytes, and C22 showed selective membrane staining of osteoclasts. Consequently, C22 was used to coat Dynabeads to affinity-purify viable human osteoclasts from osteoclastoma-derived cell suspensions. Immunocytochemical staining of inflammatory osteoarthritic synovium/granulation tissue demonstrated positivity in the majority of giant cells with MAb C22 and C27. In contrast, C35 reacted with only very occasional giant cells. Furthermore, multinucleated cells formed in long-term human bone marrow cultures demonstrated similar selective staining. C27 stained all giant cells and the majority of mononuclear cells. C22 detected only a small proportion of giant cells. In contrast to its staining on bone osteoclasts, C22 demonstrated granular cytoplasmic staining in cultured giant cells. C35 stained no cells at all in these cultures. These MAb can therefore distinguish between giant cells of various origins and authentic mature osteoclasts. Alternatively, they can recognize antigens expressed at different stages of osteoclast differentiation and therefore provide an excellent tool for the study of the human osteoclast lineage.