In mammals, it has been difficult to collect the authentic osteoclasts on a large scale. Recently we established a culture system for forming osteoclast-like multinucleated cells (MNCs) which exhibited many characteristics of osteoclasts. MNCs instead of authentic osteoclasts were used as immunogen to establish the hybridomas which secrete monoclonal antibodies against the osteoclasts by in vitro immunization. We obtained two monoclonal antibodies, HOK 1 and HOK 2. HOK 1 showed intense immunoreactivity with MNCs, mononuclear cells and putative migratory traces of MNCs on the culture dishes, but it had weak reactivity with the stromal cells. HOK 2 also showed strong reactivity with MNCs, mononuclear cells, and the very limited area of the culture dishes just facing to one side of some MNCs. The stromal cells were faintly stained with HOK 2. In th paraffin sections of tibiae, both antibodies intensely stained osteoclasts, osteoblasts and osteocytes. The bone matrix was weakly stained by HOK 2 but not by HOK 1. The present observations indicated that HOK 1 and HOK 2 could recognize the common antigen expressed on cells both involving in the bone formation and resorption. The antigens recognized by HOK 1 and HOK 2 were shown to exist on the putative traces of motile MNCs. These antibodies would be available to investigate the mechanisms of the 'bone remodeling' as a functional marker of bone cells.