Although calcitonin has been clinically utilized as a primary treatment for several metabolic bone diseases, its inhibitory effects against osteoclastic function diminish after several days owing to the calcitonin 'escape phenomenon'. We have previously found a unique cell-surface antigen (Kat1-antigen) expressed on rat osteoclasts. Here we show evidence that, in the presence of calcitonin, the Kat1-antigen is involved in osteoclastogenesis. Treatment of bone marrow cultures for forming osteoclast-like cells with anti-Kat1-antigen monoclonal antibody (mAb Kat1) provoked a marked stimulation of osteoclast-like cell formation only in the presence of calcitonin but not in its absence. Osteoclastogenesis stimulated by the receptor activator of nuclear factor kappa B (NF-kappaB) ligand/osteoclast differentiation factor was further augmented by mAb Kat1 in the presence of calcitonin. Furthermore, even in the presence of the osteoprotegerin/osteoclast inhibitory factor, mAb Kat1 induced osteoclast-like cell formation. Our current data suggest that the Kat1-antigen is a molecule that is distinct from receptor activator of NF-kappaB. The presence of the unique Kat1-antigen on cells in the osteoclast lineage appears to contribute to the fine regulation of osteoclastogenesis in vivo. Expression of this cell-surface molecule in cells in the osteoclast lineage may partly explain the mechanism responsible for the escape phenomenon.