The effects of rare earth ions on bone resorbing function of osteoclasts were studied by culturing Japanese white rabbit osteoclasts on bone slices. In order to evaluate the activity of osteoclasts, the number and surface areas of lacunae were measured by photomicrography and image analysis, and the calcium concentration in the supernatant was measured by the atomic absorption spectrometry. The lacunae morphology was observed under a scanning electron microscope. The results indicated that La3+, Sm3+ and Er3+ at the concentration of 1.00?0-5, 1.00?0-6 and 1.00?10-7mol/L and Nd3+, Gd3+ and Dy3+ at the concentration of 1.00?10-5 and 1.00?0-6 mol/L inhibited osteoclastic activity as indicated by the dose-dependent reduction in the numbers and surface areas of the lacunae (P<0.01). On the contrary, the number and surface areas of lanunae were increased and osteoclastic bone resorbing function was significantly enhanced by La3+, Sm3+ and Er3+ at the concentration of 1.00?0-8 mol/L and Nd3+, Gd3+ and Dy3+ at the concentration of 1.00?0-7 mol/L (P<0.01). Nd3+, Gd3+ and Dy3+ had no effect on osteoclastic bone resorption function at concentrations as low as 1.00?0-8 mol/L (P>0.05). It is suggested that the effects of rare earth ions on osteoclastic bone resorption are bidirectional, depending on concentrations and species. The effects of rare earth ions on bone resorbing function of osteoclasts were studied by culturing Japanese white rabbit osteoclasts on bone slices. In order to evaluate the activity of osteoclasts, the number and surface areas of lacunae were measured by photomicrography and image analysis, and the calcium concentration in the supernatant was measured by the atomic absorption spectrometry. The lacunae morphology was observed under a scanning electron microscope. The results indicated that La3 +, Sm3 + and Er3 + at the concentration of 1.00? 0-5, 1.00? 0-6 and 1.00 10-7 mol / L and Nd3 +, Gd3 + and Dy3 + at the concentration of 1.00-10-5 and 1.00-0-6 mol / L rendered osteoclastic activity as indicated by the dose-dependent reduction in the numbers and surface areas of the lacunae (P <0.01). On the contrary, the number and surface areas of lanuna were increased and osteoclastic bone resorbing function was significantly enhanced by La3 +, Sm3 + and Er3 + at the concentration of 1.00? 0-8 mol / L and Nd3 + , Gd3 + and Dy3 + at the concentration of 1.00-0-7 mol / L (P <0.01). Nd3 +, Gd3 + and Dy3 + had no effect on osteoclastic bone resorption function at concentrations as low as 1.00? 0-8 mol / L (P > 0.05). It is suggested that the effects of rare earth ions on osteoclastic bone resorption are bidirectional, depending on concentrations and species.