PURPOSE: The study was aimed to detect the differences in the cell viability and the apoptosis induction after irradiation on normal and tumorigenic cells. MATERIALS AND METHODS: The study, that was generated for two human normal cells(RHEK, HGF-1) and two human tumor cells(KB, HT-1080), was tested using MTT assay at 1 day and 3 day after irradiation and TUNEL assay under confocal laser scanning microscope at 1 day after irradiation. Single irradiation of 0.5, 1, 2, 4, and 8 Gy were applied to the cells. The two fractions of 1, 2, 4, and 8 Gy were separated with a 4 hour time interval. The irradiation was done with 5.38 Gy/min dose rate using Cs-137 irradiator at room temperature. RESULTS AND CONCLUSIONS: 1. In 3-day group, the cell viability of HGF-1 cell was significantly decreased at 2, 4 and 8 Gy irradiation, the cell viability of KB cell was significantly decreased at 8 Gy irradiation and the cell viability of HT-1080 cell was significantly decreased at 4 and 8 Gy irradiation. 2. There was significant difference between RHEK and KB cell line in the cell viability of 3-day group at 8 Gy irradiation. There was significant difference between RHEK and HGF-1 cell line in the cell viability of 3-day group at 4 and 8 Gy irradiation. 3. There was a significantly decreased cell viability in 3-day group than those in 1-day group at 2, 4 and 8 Gy on HGF-1 cell, at 4 and 8 Gy on HT-1080 cell, at 8 Gy on KB cell. 4. We could detect DNA fragmented cells only on KB cell. Number of apoptotic cells of KB cell was significantly increased at 4 and 8 Gy irradiation. However, there was no correlation between cell viability and apoptosis. 5. On all 4 cell lines, there were no differences between single and split irradiation method in cell viability and apoptosis.