The activities of 13 sesquiterpenes isolated from Tripterygium wilfordii Hook fil. var. regelii Makino were studied against herpes simplex virus type 1 (HSV-1) in vitro. Among these compounds, only triptofordin C-2 showed a selectivity index of more than 10. The compound, which could also inhibit the replication of human cytomegalovirus (HCMV), did not affect either adsorption or penetration of HSV-1 to host cells, but showed moderate virucidal activity against several enveloped viruses including HSV-1, HCMV, measles virus and influenza A virus. Triptofordin C-2 suppressed viral protein synthesis of infected cells when added at early steps of HSV-1 replication and exerted inhibition of translation of the transcripts of the immediate early genes. When acyclovir and triptofordin C-2 were evaluated in combination for antiviral activity against HSV-1 replication, additive antiviral effects were observed for this virus.