Osteoclast differentiation factor in human osteosarcoma cell line. A sensitive sandwich enzyme-linked immunosorbent assay (ELISA) for human osteoclast differentiation factor (ODF/RANKL/ OPGL/TRANCE) utilizing a polyclonal antibody that recognizes both human soluble ODF and mouse ODF in combination with a osteoclasogenesis inhibitory factor (OCIF/OPG) was developed. We can quantify the ODF level in not only human ODF (detection limit: 0.05 ng/ml), but also mouse ODF by virtue of cross-reactivity. Employing this assay system, we demonstrated that ODF is constitutively present as a membrane-bound form in both the human osteosarcoma cell lines, MG-63, HOS and SaOS-2, and the mouse osteoblastic cell line MC3T3-E1.