Aqueous and ethanol extracts of propolis were analysed phytochemically and examined for their antiviral activity in vitro. Different polyphenols, flavonoids and phenylcarboxylic acids were identified as major constituents. The antiviral effect of propolis extracts and selected constituents, e.g. caffeic acid (1), p-coumaric acid (2), benzoic acid (3), galangin (4), pinocembrin (5) and chrysin (6) against herpes simplex virus type 1 (HSV-1) was analysed in cell culture. The 50% inhibitory concentration (IC(50)) of aqueous and ethanol propolis extracts for HSV-1 plaque formation was determined at 0.0004% and 0.000035%, respectively. Both propolis extracts exhibited high levels of antiviral activity against HSV-1 in viral suspension tests, plaque formation was significantly reduced by >98%. In order to determine the mode of antiviral action of propolis, the extracts were added at different times during the viral infection cycle. Both propolis extracts exhibited high anti-HSV-1 activity when the viruses were pretreated with these drugs prior to infection. Among the analysed compounds, only galangin and chrysin displayed some antiviral activity. However, the extracts containing many different components exhibited significantly higher antiherpetic effects as well as higher selectivity indices than single isolated constituents. Propolis extracts might be suitable for topical application against herpes infection.