The great diversity of immunodeficient states and factors affecting the immune reactivity lead to the necessity of developing and testing immunomodulating preparations capable of introducing appropriate corrections [1 -3]. Use of the immunomodulating drugs in the combined therapy of oncologic diseases is a problem of current importance because all specific antitumor drugs and ionizing radiations exhibit more or less pronounced immunodepressant effect [4,5]. The action of these factors usually develops on the background of immune deficiency caused by the presence of a malignant tumor.Unfortunately, the number of immunomodulating drugs available in practice is small [3].Arbidol, or 6-bromo-5-hydroxy-4-dimethylaminomethyl-1-methyl-2-phenylthiomethyl-3-ethoxyearhydrochloride monohydrate, is a new domestic antiviral drug possessing immunomodulating properties. The drug offers good prospects in the prophylaxis and therapy of both viral diseases and disorders induced by ionizing radiation [6 -8].Levamisole, or (S)-2,3,5,6-tetrahydro-6-phenylimidazo[2, l-b]thiazole hydrochloride (also known as decaris), is an antihelminthic drug reported also to possess pronounced immunomodulating properties [9]. There is evidence that the drug can be used in oncology and medical radiology for the purpose of immune reactivity correction [10][11][12].It was therefore an important task to compare the immunomodulating activity of arbidol and levamisole under the conditions of ionizing radiation and experimental malignancy in animals.