The immunomodulating activity of arbidole was studied in cultured cells, animals, and human beings. Arbidole was shown to have effects on nonspecific defense factors, on its capacity to induce interferon and activate phagocytes in particular. Arbidole-treated patients with lower baseline immunity showed improvement in immunological parameters (in the counts of CD4 and CD8 lymphocytes, B lymphocytes, in the levels of serum immunoglobulins). Arbidol produces a high preventive and therapeutical effects in influenza A and B and other acute respiratory viral infections, prevents postinfluenza complications, reduces the incidence of exacerbations of chronic diseases in postinfluenza patients. In influenza, the therapeutical efficiency of the drug appears as decreases in intoxication, the severity of catarrhal syndrome, shorter fever and disease in general. Arbidole is beneficial for patients with second immunodeficiency, in those with recurrent herpes infection or chronic bronchitis. After arbidole treatment regimen, postoperative immunological parameters became normal in cardiac surgical patients, which suggests that the drug should be used in cardiac surgical care. The agent showed no side effects in any case.