We have explored the use of adenovirus vector-mediated gene transfer to introduce foreign genes into osteoclasts, terminally differentiated cells responsible for bone resorption. A replication-deficient adenovirus vector that contains a reporter gene encoding β-galactosidase efficiently infected human osteoclast-like cells (OCLs) derived from human giant cell tumors and mouse OCLs formed in vitro. We then constructed an adenovirus vector carrying human epidermal growth factor receptor (EGFR) cDNA (Ax1CAhEGFR) and introduced the EGFR gene into mouse OCLs. Clear induction of EGF receptor was detected in Ax1CAhEGFR-infected OCLs (EGFR-OCLs) by immunocytochemistry and immunoblotting, and EGF stimulation induced rapid tyrosine phosphorylation of several proteins including EGF receptor itself. Large vacuoles appeared in EGFR-OCLs in response to EGF treatment, and pit-forming activity by EGFR-OCLs was dose-dependently suppressed by recombinant human EGF. In addition, survival of EGFR-OCLs was prolonged by EGF. No expression of EGF receptor or effects of EGF were observed in noninfected OCLs or control vector-infected OCLs. These results suggest that adenoviral vectors are useful for modulating osteoclast function by introducing foreign genes into osteoclasts and that they will be a good means of gene therapy of metabolic bone diseases.