Triterpenoid saponins are naturally occurring sugar conjugates of triterpenes possessing various biological activities, including antiviral action. Two substances isolated from natural sources were tested against herpes simplex virus type 1 replication. They did not show evidence of cytotoxicity under antiviral test conditions. The triterpenoid saponin, isolated from a Brazilian plant (s21), represents the oleanane group and inhibited herpes simplex virus type 1 DNA synthesis. The triterpenoid saponin, isolated from a Chinese plant (s17), represents the ursane group and seemed to inhibit viral capsid protein synthesis of herpes simplex virus type 1.