Osteoprotegerin (OPG)/osteoclastogenesis inhibitory factor (OCIF) is a soluble member of the tumor necrosis factor receptor family and plays a crucial role in the negative regulation of osteoclastic bone resorption. We have immunized OPG/OCIF knockout mice with murine rOPG/rOCIF and established a panel of hybridomas producing monoclonal antibodies (mAbs) to murine rOPG/rOCIF. Utilizing the mAbs, we developed enzyme-linked immunosorbent assay (ELISA) systems: one detecting both homodimeric and monomeric forms of murine OPG/OCIF and the other detecting only dimeric form of murine OPG/OCIF. With the aid of these ELISA systems we showed that OPG/OCIF is present mainly as a monomer in murine blood. The concentration of OPG/OCIF in normal mouse sera was approximately 500 pg/ml and there was no statistical difference in the serum concentration of OPG/OCIF among genders, age, and strains. Interestingly, the concentration of circulating OPG/OCIF in mouse markedly increased during pregnancy. The result indicated that circulating OPG/OCIF plays an important role in the protection of bone from excess resorption during pregnancy in mammals.