While acyclovir, a nucleoside analogue, is widely used for herpes simplex virus type 1 (HSV-1), emergence of drug-resistant viruses due to frequent usage of this class of medicines, and their toxic side effects require exploring novel active molecules. Despite the studies on developing synthetic molecules in medical sciences and pharmacology, herbs as a natural source of biologically-active compounds remain popular. In this in vitro study, olive leaf extract (OLE) and propolis alone or in combination with acyclovir were investigated for their antiviral efficacy in HSV-1.Toxic doses of OLE, propolis, and dimethyl sulfoxide, propolis diluent, for Hep-2 (ATCC, CCL-23) cells were determined by conventional cell culture. Using "endpoint" method, the viral dose infecting half of the cell culture (TCID50) was calculated, and viral quantity was determined with Spearman-Karber method. Antiviral effects of OLE and propolis on HSV-1 were investigated by conventional cell culture and real-time cell analysis (RTCA). Combinations of the two extracts with one another and with acyclovir were evaluated by RTCA. Active substances prepared at three different dilutions were added to tubes with HSV-1 of logTCID(50): 11.5 in descending order starting from the highest non-toxic concentration, and they were left at room temperature for two different durations (one hour and three hours). The aliquots taken from the tubes were cultured in plates containing Hep-2 cells and evaluated after 72 hours. Combinations of extracts and acyclovir at concentrations at least four times lower than the lowest concentration showing antiviral efficacy against HSV-1 were cultured with Hep-2 cells in the e-plates of the xCELLigence RTCA device, measurements were obtained at 30 minute intervals, and data were recorded in real time. In the test with two different durations and at different concentrations of OLE and propolis, antiviral efficacy was observed both with one-hour and three-hour incubation at a concentration of 10 mu g/ml for propolis and 1.2 mg/ml for OLE with RTCA. The duration and concentration of the greatest decrease in viral quantity were in the first one hour and 10 mu g/ml for propolis, and in the first one hour and 1.2 mg/ml for OLE. Combination of propolis and OLE with acyclovir caused no cytopathic effects, and the combination of extracts led to delayed cytopathic effect. According to these results, propolis and OLE, alone and in combinations with acyclovir, have antiviral efficacy against HSV-1. These agents may reduce the dose and side effects of acyclovir in case of co-administration since they exert their effects through a different mechanism than acyclovir,possibly through direct virucidal activity, inhibition of virus internalization or viral inhibition in early stages of replication (inhibition of adsorption/binding of viral particles to the cell). These extracts that do not require conversion to active form have the potential to reduce infectivity in oral lesions, prevent spread, and be used in the topical treatment of acyclovir-resistant HSV infections, particularly in immunocompromised patients. However, in vivo studies should be conducted to determine their medicinal properties and potential toxicities. These results should be supported by further comprehensive studies and the efficacy against other viruses should also be investigated.