Apoptosis is induced by radiation, administered at different dose rates of 3-60 Gy/h, in rat embryo cells transfected with a c-myc oncogene (REC:myc(ch1)) or with a c-Ha-ras oncogene (REC:ras(ch1)). Apoptosis is evaluated in terms of altered morphology, chromatin condensation and DNA fragmentation. The apoptotic dose response of REC:myc(ch1) rises steeply at low doses (to about 40% at 5 Gy), and reaches a plateau at high doses (of about 60% at > 15 Gy). In comparison with REC:myc(ch1), the REC:ras(ch1) is much less susceptible, with a maximum apoptotic fraction of about 10%. Interestingly, radiation-induced apoptosis is nearly dose-rate independent. In parallel, we assessed radiation-induced cell-killing as assayed by colony-formation. In contrast to that observed for apoptosis, the dose response of colony-formation is strongly dependent on dose rate. Cell surviving fraction measured at 3 Gy/h decreases exponentially with dose, with REC:myc(ch1) exhibiting a steeper slope than REC:ras(ch1). Thus, the different low-dose-rate radiosensitivity of the two cell lines may in part be due to their different susceptibility to radiation-induced apoptosis. Taken together, these findings suggest that radiation-induced apoptosis contributes significantly to the initial (shoulder) region of acute dose-rate survival curves of susceptible cells, and may have implications for fractionated and low dose rate radiotherapy.