Although it has been known that TGF-${eta}1$ acts as a crucial cofactor in osteoclast differentiation, its mode of action is still unclear. In the present study, we studied the effect of TGF-${eta}1$ on the differentiation of osteoclast depending on the developmental stages. Murine bone marrow cells were induced to differentiate into mature osteoclasts in the presence of receptor activator of NF-${kappa}B$ ligand (RANKL) and macrophage colony stimulating factor (M-CSF). In the early stage of the differentiation TRAP(-) mononuclear precursor cells were obtained from nonadherent M-CSF dependent bone marrow cells, which further differentiated into mature osteoclasts. TGF-${eta}1$ stimulated osteoclast differentiation, which was stronger when cells were stimulated by TGF-${eta}1$ in the early stage than the later differentiation. TGF-${eta}1$ increased the expression of RANK and synergistically stimulated RANKL-induced activation of NF-${kappa}B$ MAP kinase in TRAP(-) mononuclear precursor cells. These results suggest that activation of osteoclast differentiation by TGF-${eta}1$ may be ascribed to the both increased expression and activation of RANK in the osteoclast differentiation, especially in the early stage of differentiation.