The study of the activity of arbidol against epidemic influenza A and B virus strains (2002-2005) in the cultured MDCK cells showed the higher sensitivity of enzyme immunoassay than that of hemagglutination test. The influenza A virus strains tested, including those resistant to rimantadine (5 microg/ml), were sensitive to arbidol (10 microg/ml). The population of influenza B virus strains was heterogeneous in this indicator, 43% of the strains being less sensitive to arbidol. There was an increase in the number of rimantadine-resistant influenza A(H3N2) virus strains (10-18%) in our country during 3 epidemic seasons. The sequencing analysis of protein M2-endoding gene revealed the amino acid replacement of serine by asparagine in position 31, which is characteristic of rimantadine-resistant strains. Arbidol in combination with rimantadine potentiated the effect of viral reproduction in the cultured cells, as compared with the effect produced by the same concentrations of the drugs used alone.