Data derived byBarendsen (1961) on the damage to the reproductive capacity of human cells in tissue culture by ionizing radiation have been subjected to probit analysis. There is no conflict with the hypothesis that the fractions of surviving cells and the log oxygen concentration can be adequately represented by the equation of a cumulative normal curve between the limits (1-P M ) r and (1-P N ) r . (1-P M ) r represents the fraction of cells surviving a radiation doser applied in the presence of 100% oxygen and (1-P N ) r represents the fraction surviving in the absence of oxygen. In contradiction to expectation the probit regression lines of the two series of tests are not parallel.