A sufficient dose of ultraviolet light (2,0537 A) inactivates most microorganisms. Exposure of inactivated cells to suitable visible light results in the recovery of a large portion of the cells from their otherwise fatal ultraviolet-induced injury. The discovery of light-induced recovery (Kelner, 1949) and its confirmation for bacteriophage by Dulbecco (1949) gives us fresh hope for solving the fundamental radiobiological problems of the lethal and mutagenic action of ultraviolet radiation. The effect of reactivating light will be referred to in this paper as photoreactivation.3 The clear-cut and sweeping nature of photoreactivation is illustrated in figure 1. Numerous workers have reported on the "antagonism" of various radiations to ultraviolet light (see review by Prat, 1936). However, the effects noted have usually been so small, and many experiments so undecisive, that their significance has been understandably overlooked. Perhaps the most pertinent work was that of Whitaker (1942), w0-ho showed that the ultraviolet-induced lengthening of the lag phase in Fucus eggs was counteracted in part by illumination wvith wvhite light. Unfortunately, the effect was comparatively slight, and the phenomenon was apparently not investigated further. That some ultraviolet-irradiated cells may recover if stored in suspension after irradiation has been observed by many (Hollaender and Emmons, 1941; Roberts and Aldous, 1949; see Kelner, 1949, for other references). The degree of recovery in stored suspensions has been relatively small, and since the possible reactivating effect of light from the room has not been controlled in such experiments, the data must be re-evaluated.