PURPOSE: To investigate the effects of irradiation on the phenotypic expression of the MC3T3-E1 osteoblastic cell line, particularly on the expression of osteocalcin and osteopontin. MATERIALS AND METHODS: Cells were irradiated with a single dose of 0.5, 1, 4, and 8 Gy at a dose rate of 5.38 Gy/min using a cesium 137 irradiator. After the specimens were harvested, RNA was extracted on the 3rd, 7th, 14th, and 21st day after irradiation. The RNA strands were reverse-transcribed and the resulting cDNAs were subjected to amplification by PCR. RESULTS: The irradiated cells demonstrated a dose-dependent increase in osteocalcin and a dose-dependent decrease in osteopontin mRNA expression compared with the non-irradiated control group. The amount of osteocalcin mRNA expression decreased significantly at the 3rd day after irradiation of 0.5, 1, 4, and 8 Gy, and also decreased significantly at the 3rd, 14th, and 21st day after irradiation in the 8 Gy exposed group compared with the control group. The degree of osteopontin mRNA expression increased significantly at the 7th day after irradiation of 0.5, 1, 4, and 8 Gy. CONCLUSION: These results showed that each single dose of 0.5, 1, 4, and 8 Gy influenced the mRNA expression of osteocalcin and osteopontin associated with the calcification stage of osteoblastic cells, suggesting that each single dose affected bone formation at the cell level.