In experiments on mice and rats the effect was studied of a single administration of aqueous extract from chlorococcal freshwater algae - Ivastimul - on their radiosensitivity. After injection of this substance the number of haemopoietic stem cells in the bone marrow and spleen of mice (CFUs) increased, as did their survival after irradiation. Irradiation with a lethal dose of gamma rays 24 hours after the injection of Ivastimul is survived by a larger number of treated mice and rats than untreated ones. On the first day after the administration Ivastimul protects mice against brief and prolonged action of irradiation. The protective effect of Ivastimul was observed after intraperitoneal, intramuscular and subcutaneous administration.