Growth curve of irradiated MC3T3-E1 cells assessed by MTT assay. MC3T3-E1 cells were exposed to 0, 0.1, 0.5, 1.0 and 2.0 Gy of X-ray irradiation respectively. The effect of irradiation on cell growth was assessed by MTT assay at 24-h intervals until day 8 after been seeded. Optical density (OD) was measured at 495 nm and data were expressed as the mean ± SD of triplicate experiments (n = 3). From day 4 to day 8, relative proliferation rate of the cells exposed to 2.0 Gy was significantly lower than those of the control group (0 Gy) (* p < 0.05, ** p < 0.01). On day 8, relative proliferation rates of the cells exposed to 0.5 and 1 Gy were lower than those of the control group and 0.1 Gy group(* p < 0.05).