Using a stopped-flow mixing and pulsed irradiation apparatus, a study has been made of the decay, to a harmless form, of radiation-induced species which would otherwise be lethal to spores on contact with oxygen. Aqueous suspensions of Bacillus megaterium spores were irradiated with 600 krad of electrons given over approximately 1 s; at various times after irradiation oxygen in solution was added. As the interval between irradiation and introduction of oxygen increased, the fraction of spores surviving increased. For spores irradiated in a deoxygenated condition the decay of the potentially lethal species, reflected by this change in survival, proceeded as if two parallel first-order reactions with half-lives of 9 and 120 s operate. In contrast, for spores equilibrated with oxygen and then irradiated, the decay is described by a single first-order expression with an associated half-life similar to that of the faster of the two reactions operating in anoxia.