The antiviral effect of rimantadine and isoprinosine applied in combination against fowl plague virus (FPV) in cell cultures of chick embryo fibroblasts and in experimental influenza infection in mice has been studied. Isoprinosine does not inhibit the reproduction of FPV when given alone and does not increase the antiviral activity of rimantadine. After oral application of both substances according to an appropriate scheme in mice infected with influenza virus A/Aichi (N3N2), an increase in the antiviral effect with an index of protection by 12% higher than the theoretically calculated one for additive effect was established. The results suggest that the combined effect is synergic.