Objective:To study the effect of endogenous parathyroid hormone (PTH) on osteoclasts (OCs) cultured in vitro. Methods:Sixteen PTH+/+ and sixteen PTH-/- mice with 8-week-old were used in this study. Bone marrow stem cells obtained from mouse femurs were isolated and cultured with macrophage colony-stimulating factor (M-CSF,30 ng/ml) and different concentration of receptor activator of NF-κB ligand (RANKL,50 ng/ml as low concentration group or 100 ng/ml as high concentration group) in culture dishes containing 10 ng/ml M-CSF for 24 h. The tartrate-resistant acid phosphatase (TRAP) staining was performed to observe and count cell morphology and TRAP-positive osteoclasts numbers at 40 or 100 magnified visual field after 6-day and 9-day culturing. Results:M-CSF and RANKL induced bone marrow stem cells to form TRAP-positive OCs. TRAP-positive OCs numbers in the same visual field decreased after 9-day culturing compared with that after 6-day culturing,but OCs volume after 9-day culturing became more larger and had more nuclei than after 6-day culturing. In the same concentration of RANKL,no changes were observed in the numbers of TRAP-positive OCs between the PTH+/+ groups and the PTH-/- groups. In contrast,either in the PTH+/+ or the PTH-/- groups,high concentration of RANKL (100 ng/ml) induce more TRAP-positive OCs. Conclusion:Endogenous PTH had no effect on the numbers of osteoclasts cultured in vitro in the presence of M-CSF and RANKL. High concentration of RANKL(100 ng/ml) increased the TRAP-positive osteoclast numbers compared with low concentration (50 ng/ml).