The authors studied the effect of various DDP applications (sodium salt of dimethyl-dioxopiperazine thiosulfonate) in a dose of 45 mg/kg w. administered 10 min before irradiation, on the survival of female mice C57 and (CBA x C57BL/F1 after a single whole-body exposure to a sublethal dose of gamma rays (LD50/5 = magnitude of 15.495 +/- 0.781 magnitude of Gy and LD50/30 = magnitude of 7.864 +/- 0.018 magnitude of Gy). The depth of DDP protection effect on the haemopoietic stem cells was studied using the method of endogenous spleen colonies (ESC) of haemopoietic tissue. DDP radioprotective effect was also demonstrated after s. c. and i. m. application of this substance 10 min before 6 Gy of gamma radiation; it was shown by a prolongation of the survival time of experimental animals and by an increase in the number of individuals surviving till the 30th day after irradiation. Higher ESC numbers in the animals with DDP protection demonstrate survival of higher number of the haemopoietic stem cells and indicate that DDP is the biological active substance and contributes to the repair of haematopoiesis damaged by radiation and to the survival of irradiated animals.