Insulin-like growth factor-I (IGF-I) and IGF-II have powerful, well defined effects on osteoblastic cells, stimulating their proliferation and inducing collagen synthesis, but the role of IGF-I and -II in modulating osteoclast differentiation and activity remains unclear. We first examined the bone-resorptive effects of IGF-I and IGF-II by assessing 45Ca2+ release from neonatal mouse calvarial bones. Both IGFs dose dependently stimulated bone resorption, with an EC50 of 8 x 10(-9) M for IGF-I and 2 x 10(-8) M for IGF-II. We then tested the effects of the IGFs on bone resorption by rat isolated osteoclasts cultured on ivory slices. Neither IGF-I nor IGF-II stimulated isolated osteoclast activity. However, in the presence of either primary mouse osteoblasts or human osteosarcoma MG 63 cells, both IGFs enhanced osteoclast resorptive activity, with an EC50 of 5 x 10(-10) M for IGF-I and 10(-9) M for IGF-II. Stimulation was not mediated by BALB/c/3T3 cells, a nonosteoblastic cell line. The effects of the IGFs were blocked by alpha IR-3, an antibody to the type I IGF receptor, but not by beta-galactosidase, a lysosomal enzyme that competes with IGF-II for the type II IGF receptor. We then examined the effects of the IGFs on the formation of osteoclast-like multinucleate cells (MNCs) in mouse bone marrow cultures. IGF-I and -II dose dependently increased the number of tartrate-resistant acid phosphatase (TRAP)-positive MNCs, although their effects were less than that of 1,25-dihydroxyvitamin D3 (a hormone that induces osteoclast differentiation). No TRAP-positive MNCs appeared in the absence of these hormones. Like authentic osteoclasts, the TRAP-positive MNCs formed in response to IGF-I and -II bound [125I]salmon calcitonin. When mouse bone marrow cells were cultured on ivory slices in the presence of either IGF-I or IGF-II for 10 days, numerous resorption lacunae were formed. beta-Galactosidase had no effect on IGF-mediated osteoclast formation. These results are strong evidence that both IGF-I and IGF-II stimulate bone resorption in vitro by enhancing osteoclast formation and function. Our data also suggest that the IGFs act through the intermediary of osteoblastic cells to stimulate osteoclast activity and that the type I, but not the type II, IGF receptor is involved in their responses. We propose that the local production of IGF-I and IGF-II may modulate both osteoblast-osteoclast interactions and osteoclast formation and play an important role in bone remodeling.