Although the action of insulin-like growth factor-I (IGF-I) on bone formation has been extensively investigated, the effect of the factor on bone resorption is little known. We first examined the effect of IGF-I on bone resorption by preexistent osteoclasts by using unfractionated bone cells cultured on dentin slices. IGF-I had a dose-related effect of stimulating bone resorption by preexistent osteoclasts, whereas IGF-II did not. When IGF-I was added to cultures of bone cells after preexistent osteoclasts had degenerated on the dentin slices, IGF-I increased the number of osteoclastic multinucleate cells (MNCs) with tartrate-resistant acid phosphatase activity. Moreover, IGF-I augmented the area of pits produced by newly formed osteoclasts. These results suggest that IGF-I directly or indirectly stimulates osteoclast recruitment and activation. Therefore, we next examined the direct effect of IGF-I on osteoclastic MNC formation by using hemopoietic blast cells. In the presence of 1,25-dihydroxyvitamin D3, IGF-I, like granulocyte-macrophage colony-stimulating factor (GM-CSF), dose-dependently increased the number of TRAP-positive MNCs. This stimulatory effect of IGF-I was additive with that of GM-CSF. Both IGF-I and GM-CSF supported the survival of the blast cells, indicating that IGF-I as well as GM-CSF are supporting factors for osteoclast differentiation. In addition, the blast cells possessed high affinity binding sites for IGF-I, with a Kd of 0.8 nM. These data, thus, indicate that IGF-I stimulates osteoclastic bone resorption through its direct or indirect action of supporting the generation and activation of osteoclasts.