The main objective of this study was to evaluate the antiviral properties of the glyproline Selank in both in vitro and in vivo against the influenza virus strain A/Aichi 2/68 (H3N2). The pronounced antiviral effect of the agent was detected in both systems. Selank added to the cell culture 24 hours before inoculation (a preventive use scheme) showed the highest efficiency, by completely suppressing viral reproduction. The in vivo studies also demonstrated that the highest survival of laboratory animals was observed when the agent was administered by the prevention scheme. The use of Selank in vivo induced the gene expression of interferon-alpha (IFN-alpha), without affecting that of interleukin (IL)-4, IL-10, or tumor necrosis factor-alpha (TNF-alpha). The findings suggest that the mechanism of the antiviral action of Selank may be due to its ability to modulate Th1/Th2/Treg cytokine equilibrium both directly and indirectly via the central nervous system. This is particularly promising if that the agent is synthesized on the basis of an endogenous peptide and that it has no negative effects is kept in mind.