BACKGROUND: The herb formulation Deva-5 is used in traditional medicine to treat acute infectious diseases. Deva-5 is composed of five herbs: Gentiana decumbens L., Momordica cochinchinensis L., Hypecoum erectum L., Polygonum bistorta L., and Terminalia chebula Retz. Deva-5 and its five components were investigated for in vitro antiviral activity against avian influenza A virus subtype H3N8. METHODS: The water extracts of the herbal parts of G. decumbens, H. erectum and P. bistorta, the seeds of T. chebula and M. cochinchinensis and Deva-5 were prepared by boiling and clarified by low-speed centrifugation and filtration. To assess the antiviral properties, avian influenza virus isolate A/Teal/Tunka/7/2010(H3N8) was incubated at 37°C for 30 min in the presence and absence of the extracts of five plants and DEVA-5 in various concentrations. Subsequently, the concentration of infectious virus in each sample was determined by plaque assays. Neutralisation indexes and 90% plaque reduction concentrations were estimated for each extract, and the significance of the data was evaluated using statistical methods. RESULTS: The extracts of G. decumbens, H. erectum, P. bistorta and Deva-5 demonstrated no significant toxicity at concentrations up to 2%, whereas extracts of T. chebula and M. cochinchinensis were well-tolerated by Madin-Darby canine kidney cells at concentrations up to 1%. The extracts of H. erectum, M. cochinchinensis and T. chebula reduced the titre of A/Teal/Tunka/7/2010 (H3N8) by approximately five-fold (p???0.05). The other three extracts did not significantly reduce the infectivity of the virus. The plaque reduction neutralisation tests revealed that none of the extracts tested were able to inhibit formation of plaques by 90%. However, three extracts, H. erectum, T. chebula and M. cochinchinensis, were able to inhibit formation of plaques by more than 50% at low dilutions from 1:3 to 1:14. The T. chebula extract had a concentration-dependent inhibitory effect. CONCLUSIONS: For the first time, the consistent direct antiviral action of the extracts of H. erectum, T. chebula and M. cochinchinensis was detected. These extracts significantly reduced the infectivity of influenza A virus H3N8 in vitro when used at high concentrations (0.5-1%). However, Deva-5 itself and the remainder of its components did not exhibit significant antiviral action. The results suggest that H. erectum, T. chebula and M. cochinchinensis plants contain substances with direct antiviral activity and could be promising sources of new antiviral drugs.