The aim of this study was to determine the inhibitory action of alantolactone, a gradient of traditional Chinese medicine Inulae Radix (Tu-Mu-Xiang), on herpes simplex virus 1 (HSV-1). African green monkey kidney cells (Vero cells) were infected with HSV-1 and the protective effects of alantolactone on Vero cells were examined. At concentrations of 10(-6), 10(-7), and 10(-8) g/mL, alantolactone did not have a marked harmful effect on the viability of Vero cells according to an MTT assay. Based on the cytopathic effect (CPE) and MTT assays, alantolactone at these concentrations exhibited antiviral action and protected cells from being damaged by HSV-1. Results indicated that alantolactone had potent anti-HSV-1 action and provided evidence for use of Inulae Radix in the treatment of HSV-1 infection.