Introduction. Active circulation of pandemic influenza and new variants of influenza H3N2 strains requires monitoring of antiviral efficacy of drugs permitted for influenza therapy in the Russian Federation. Purpose. Assessment of antiviral efficacy of «Kagocel» substance against influenza viruses H1N1, H1N1pdm09 and H3N2 in vitro. Material and methods. Cytotoxic effect of «Kagocel» substance on MDCK cells had been determined by stained with MTS. Antiviral efficacy of «Kagocel» substance against influenza infection has been studied in vitro in the culture of MDCK cells infected with influenza virus strains: A/Puerto Rico/8/34 (H1N1), А/California/7/2009 (H1N1)pdm09, А/Hong Kong/1/68 (H3N2) and А/ Hong Kong/4801/2014 (H3N2). The antiviral activity of «Kagocel» substance was tested by its effect on the infectious titer of the influenza viruses and on its impact on the expression level of viral antigens in the enzyme immunoassay test system. Results. «Kagocel» substance had low toxicity for MDCK cells. «Kagocel» inhibited the infection titer of influenza virus strains A/Puerto Rico/8/34 (H1N1), А/California/7/2009 (H1N1)pdm09, А/Hong Kong/1/68 (H3N2) and А/ Hong Kong /4801/2014 (H3N2) in the MDCK cell culture with equal efficacy. Study of the impact of «Kagocel» substance on the expression level of viral antigens by ELISA also revealed its antiviral efficacy for all tested strains. Dose dependence was observed from concentration of substance and from infective dose of virus. Discussion. Effective suppression of the reproduction of influenza virus strains A(H1N1), A(Н1N1)pdm09 and A(H3N2) in the different sublines of MDCK cells with «Kagocel» was shown by the different methods. These results give the possibility to suggest that along with the ability to induce interferons, «Kagocel» can impact on the reproduction of influenza virus, but the further research is needed. Conclusion. «Kagocel» substance effectively inhibits the reproduction of influenza virus strains A(H1N1), A(Н1N1)pdm09 and A(H3N2) in vitro. At the same time, the selectivity index is quite high. © 2019 Izdatel'stvo Meditsina. All rights reserved.