The water-soluble extract of Spirulina platensis achieved a dose-dependent inhibition of the replication of herpes simplex virus type 1 (HSV-1) in HeLa cells within the concentration range of 0.08–50 mg/mL. This extract proved to have no virucidal activity and did not interfere with adsorption to host cells. However, the extract affected viral penetration in a dose-dependent manner. At 1 mg/mL the extract was found to inhibit virus-specific protein synthesis without suppressing host cell protein synthesis if added to the cells 3 h before infection. In an in vivo experiment food containing the extract effectively prolonged the survival time of infected hamsters at doses of 100 and 500 mg/kg per day.