In intensity-modulated radiation therapy (IMRT) and stereotactic irradiation using a linear accelerator, radiation is administered intermittently and one treatment session often requires 30 min or a longer time. The purpose of the present study was to investigate the effect of fractionation and dose per fraction on cell killing by irradiation in intermittent exposure. Murine EMT6 and SCCVII cells were used. The cells were irradiated to a total dose of 8 Gy in 2, 5, 10, 20 and 40 fractions over 15, 30 and 46 min. The cells were also given 8 Gy in a single fraction over 15, 30 and 46 min using lower dose rates (continuous prolonged radiation groups). As compared with the control group receiving a single dose of 8 Gy at 1.55 Gy/min, the cell surviving fraction generally increased in groups receiving fractionated or continuous prolonged radiation. There was a general trend for cell survival to increase with the fraction number up to 20 or 40 fractions in both cell lines. The effects of IMRT and linear accelerator radiosurgery given over 15 min or longer may be less than those of 1- or 2-fraction irradiation. There was a trend for radiation effect to decrease with fraction number.