Relations based on the microdosimetric-kinetic (MK) model are presented that describe killing of mammalian cells by protracted continuous exposure to ionizing radiation of varying linear energy transfer quality (LET) at constant dose rate. The consequences of continuous irradiation exposure actually consisting of a discontinuous sequence of events corresponding to passage of each high-energy particle through or near the cell are incorporated into the model. The derived relations are applied to protracted irradiation experiments of Amdur and Bedford to determine the rate of repair of potentially lethal lesions. It is found that as the dose rate becomes less than about 5 Gy per hour the repair rate decreases significantly with decreasing dose rate. This suggests that repair function in these cells is induced and maintained in response to the intensity of irradiation. Clinical and radiation protection implications of this finding are noted.