The influence of X-radiation on activity of lysosomal enzymes (D, L, H cathepsins) in rat spleen tissue and in inoculated rat sarcoma 45 has been investigated. Intact rats and rats with tumors were subjected to whole-body and sarcoma 45 to local irradiation with doses of 0.155 C/kg and 0.31 C/kg in conditions of breathing gas hypoxic mixture containing 90% of nitrogen and 10% of oxygen (GHM-10). The combined exposure to radiation and GHM-10 was shown to produce a certain protective action (e.g. normalized cathepsin activity) in the spleen. In the tumor tissue the protective effect of GHM-10 was absent.