A brief explanation of the dangers inherent in the indiscriminate use of ionising radiations and a description of the modality of action of cell radiations are followed by an account of the significance of lesions of the genetic inheritance. Data obtained from the irradiation of human chromosomes are also presented. In these experiments, the effects of scalar doses on in vitro peripheral blood cultures and 'in vivo' on patients irradiated in the course of treatment of the lumbosacral area were studied. The radioprotective activity of various pharmacological agents was measured by means of an original method and the results obtained are discussed.