Total body irradiation (TBI) followed by bone marrow rescue is being increasingly used in the systemic treatment of acute leukaemia and some solid tumours such as neuroblastoma. Typically, these neoplasms are radiosensitive with little or no shoulder on the in vitro survival curve (n approximately equal to 1.0, Do approximately equal to 1.0 Gy). In such cases, fractionated or low-dose-rate TBI should allow preferential sparing of normal tissues. With the appropriate choice of dose rate, low-dose-rate TBI should, in principle, be radiobiologically equivalent to fractionated TBI. Calculations based on an extension to the linear quadratic model suggest that extremely low dose rates (e.g., approximately equal to 0.5 Gy h-1) might be required for equivalence to conventionally fractionated schedules. Such low dose rates would require very long treatment times (e.g., approximately equal to 24 h), which renders them impractical. For cell survival parameters of typical radiosensitive neoplasms the effects of proliferation do not alter this conclusion. These studies suggest that fractionated TBI (with high dose rates) is preferable to low-dose-rate therapy for neoplasms such as leukaemia and neuroblastoma.