Based on chemotaxonomic and ethno-pharmacological criteria, three Mexican plants (Jatropha dioica, Salvia texana and S. ballotaeflora) were studied for in vitro activity against HSV-1 and HSV-2. Hydro-methanolic extracts were initially evaluated for their toxicity to Vero cells. Both Salvia species displayed cytotoxicity at the lowest dose (125 microg/mL). The J. dioica extract showed only negligible cytotoxicity (CC50 644 microg/mL). Its anti-HSV activity was evaluated using the plaque reduction assay with HSV-1 and HSV-2 (from clinical isolates) infected Vero cells. The hydro-methanolic extract of J. dioica showed IC50s of 280 and 370 microg/mL against HSV-1 and HSV-2, respectively. The n-hexane liquid-liquid partition of J. dioica extract contained the majority of the active principle(s) with IC50 values of 300 and 270 microg/mL for HSV-1 and HSV-2, respectively. Bioassay-guided isolation led to the known diterpene, riolozatrione.