Saikosaponin d, isolated from the roots of Bulpleurum falcatum L. was investigated for both its inactivating effects on some viruses and its antiviral effects against the viruses in vitro. Saikosaponin d at a concentration of more than 5 microM had direct inactivating effects on both measles virus and herpes simplex virus after incubation of the viruses with the agent for more than 10 min at room temperature. In contrast, exposure of poliovirus to even 500 microM of saikosaponin d resulted in no loss of infectivity, while the same concentration of saikosaponin d induced complete loss of infectivity in both measles virus and herpes virus. In addition, saikosaponin d was ineffective against the replication of measles virus, herpes virus, and polio virus at a concentration of 0.1 microM, whereas saikosaponin d did not induce an inhibitory effect on the growth of Vero cells, when Vero cells were treated with saikosaponin d 24 h before the inoculation (pretreatment) and immediately or 24 h after the infection of the viruses (post-treatment).