The essential oil obtained in toto from Santolina insularis was investigated for its antiviral activity on herpes simplex type 1 (HSV-1) and type 2 (HSV-2) in vitro. The IC(50) values, determined by plaque reduction assays, were 0.88 and 0.7 microg/ml for HSV-1 and HSV-2, respectively, while the CC(50) determined by the MTT test on Vero cells was 112 microg/ml, indicating a CC(50)/IC(50) ratio of 127 for HSV-1 and 160 for HSV-2. Results obtained by plaque reduction assays also indicated that the antiviral activity of S. insularis was principally due to direct virucidal effects. Antiviral activity against HSV-1 and HSV-2 was not observed in a post-attachment assay, and attachment assays indicated that virus adsorption was not inhibited. Up to 80% inhibition of HSV-1 was achieved at the concentration of 40 microg/ml by yield reduction assay. Furthermore, reduction of plaque formation assays also showed that S. insularis essential oil inhibits cell-to-cell transmission of both HSV-1 and HSV-2.