Experiments are described in which HeLa cells were cultured quantitatively in vitro and irradiated with cobalt-60 gamma rays at dose rates of 2.37, 16.9 and 44.9 rads per minute. Survival curves were obtained for each of the 3 dose-rates relating the fraction of cells retaining their reproductive integrity to the total radiation dose. Values of the 37% dose slope, in order of decreasing dose-rates were 181, 205 and 259 rads. From these results it was concluded that reported variations in LD50(30) for small mammals irradiated with X- and gamma rays at different dose rates may be fully accounted for at the cellular level.