- Background: Propolis, a natural product from beehives, comprises a complex of chemicals, the most important group being flavinoids, which play a role in antiviral protection. Objectives: To test the inhibitory effect of propolis extract against herpes simplex viruses in vitro and in vivo. Methods: In vitro: propolis was added to Vero cells at various times and concentrations before, at or after infection with HSV-1. In vivo the effect of propolis was tested in newborn rats infected s.c. or i.p. and on rabbit corneas infected with HSV-1. Results: In vitro. 0.5% propolis extract caused 50% inhibition of HSV infection. There was indirect evidence for a strong interaction between the propolis extract and the surface of the Vero cells, but there was no direct interaction with HSV-1 particles. Administration of propolis before or at the time of infection yielded the most significant inhibitory effect, but even when 10% propolis extract was added 2 hours post-infection it gave 80-85% protection. In vivo as little as 5% propolis prevented the appearance and development of symptoms of local and i.p. HSV-1 infection in rats and of corneal HSV-1 infection in rabbits. There were no cytotoxic effects at a concentration of 10% in vitro or 20% in vivo. Conclusions: The potent antiviral activity of propolis against HSV-1 infection in vitro and In vivo is probably due to prevention of virus absorption into the host cells and/or inhibition of an internal step(s) during the viral replication cycle.