Experiments on strains of yeast with different genetic backgrounds were done to evaluate the kinetics of inactivation and mutation induction by X-radiations. A system of forward mutation induction in five loci was used as a specific mutation rate of 0.14-10-minus 8 times locus times rad was evaluated for the wild type. From a comparison of observations with wild type and radiation-sensitive strains, it may be assumed that, in this yeast, mutations are mainly the result of a repair-active process. The range of genotypic and phenotypic influence upon the specific locus mutation rate was evaluated with appropriate biological material and experiments.