The biological activity of cycloferon (CF) used in interferon manufacture has been investigated for its therapeutic and prophylactic properties which may be used for radiotherapy of glioma. It was demonstrated in experiments on rats that use of CF is most effective at an early stage of irradiation due to decrease in tumor mass which does not occur in case of irradiation alone. When used at a later stage, CF caused certain hematological indices of immunity to rise. CF did not produce an antitumor effect of its own. It successfully competed with Leukomax when used to bring blood indices in irradiated tumor-bearing animals back to normal.