There have been several lines of evidence that parathyroid hormone (PTH) stimulates production of insulinlike growth factor I (IGF-I) in bone and that IGF-I stimulates osteoclast formation. Thus, the present study was performed to clarify the possible role of IGF-I in PTH-stimulated osteoclastlike cell formation and the role of PTH-responsive dual signal transduction systems (cyclic [c] AMP-dependent protein kinase [PKA] and calcium/protein kinase C [PKC]) in its mechanism. Treatment with anti-IGF-I antibody (1-10 mg/ml) partially but significantly blocked hPTH-(1-34)-stimulated osteoclastlike cell formation in unfractionated mouse bone cell cultures, although it did not affect osteoclastlike cell formation stimulated by 1,25-dihydroxyvitamin D 3 . Rp-cAMPS (10 04 M), a direct PKA inhibitor, as well as two types of PKC inhibitors, H-7 (10 mM) and staurosporine (3 nM), and dantrolene (10 05 M), an inhibitor of calcium mobilization from intracellular calcium stores, all significantly blocked PTH-stimulated osteoclastlike cell formation. Anti-IGF-I antibody (3 mg/ml) significantly blocked osteoclastlike cell formation stimulated by 10 04 M dbcAMP, 10 04 M Sp-cAMPS, a direct PKA activator, and 10 05 M forskolin in mouse bone cell cultures. Dibutyryl cAMP, forskolin, and hPTH-(1-34) significantly stimulated mRNA expression of both IGF-I and IGF-binding protein 5 (IGFBP-5) in these cultures, but neither 10 07 M PMA, a PKC activator, nor 10 07 M A23187 did. Moreover, anti-IGF-I antibody significantly blocked osteoclastlike cell formation stimulated by the conditioned medium from MC3T3-E1 cells pretreated with 10 08 PTH-(1-34), which induced IGF-I and IGFBP-5 mRNA expression in these cells. In conclusion, the present study indicates that IGF-I mediates osteoclastlike cell formation stimulated by PTH and that the PKA pathway is involved in its mechanism. However, IGF-I does not seem to be the sole effector molecule to be active in this system.