Varying concentrations of the selenium biological compound "Selena" were tested in animals subjected to the action of extremal factors simulating those that affected the population at the territories radiocontaminated as a result of the catastrophe at the Chernobyl NPS (131I and external irradiation in falling down doses). The death rate in the group of animals that were not given "Selena" comprised 30% during 6 months after irradiation, their body mass was 120 g lower as compared to the animals in the group of biological control; among the animals given 0.03 mg of selenium/day the death rate was 4 times lower.