SummaryThe survival fractions of diploid and haploid strains of S. cerevisiae after ultraviolet irradiation (2537 A.U.) have been obtained, compared, and modified by photoreactivation following the initial irradiation. The haploid survival follows a sigmoid curve and is smaller than the diploid at the same dose. The dose reduction principle (which can be successfully applied to the data) yields a reduction factor that appears substantially the same for photoreactivation of both strains. The data appear consistent with the model proposed by Novick and Szilard.