AbstractSomewhat conflicting data from earlier experiments by ourselves and others suggested that under certain conditions, survival curves for the reproductive capacity of mammalian cells irradiated at ultra-high dose-rates (nanosecond-to-microsecond pulses) with X rays or high-energy electrons exhibited an inflected “hockey-stick” shape. Other workers failed to observe such “hockey-stick” survival curves when using apparently similar radiation conditions.Using a 400 keV electron beam with a pulse duration of about 3 ns, dense monolayers of HeLa cells were irradiated attached to plastic Petri dishes, or dense monolayers of P.388 murine ascites leukaemia cells were irradiated suspended on membrane filters. The cells were either aerobic, hypoxic, or in a 0·35 per cent oxygen/nitrogen atmosphere at the time of irradiation. Survival of HeLa cells was assessed by the ability to form macroscopic clonesin vitro; survival of P.388 leukaemia cells was assessedin vivoby tumour “takes” from serial dilutions of the tumour cells in isologous mice.Up to doses of about 1,500 rads (surviving fraction ∼10−3for HeLa, ∼10−6for P.388) the survival curves were uninflected, as were the curves for irradiation in nitrogen up to doses of about 3,500 rads. OER values were>2·5 for both HeLa cells and the P.388 leukaemia. However, irradiation in 0·35 per cent oxygen/nitrogen produced an inflected “hockey-stick” survival curve which, above doses of a few hundred rads, was effectively parallel to the curve for cells irradiated in nitrogen.The present results clearly indicate that “radiobiological hypoxia” can be produced in mammalian cells by irradiation with nanosecond pulsesonlyin cells already at a reduced level of oxygen. This makes the prospect for the use of these short radiation pulses for radiotherapy considerably less attractive than had been suggested by our earlier studies.