From the root of Limonium sinense (Girard) Ktze a new (2R,3S)-3,5,7,4'-tetrahydroxy-3',5'-dimethoxyflavanone was isolated and named isodihydrosyringetin (3), together with nine other known compounds, (-)-epigallocatechin 3-O-gallate (1), samarangenin B (2), myricetin (4), myricetin 3-O-alpha-rhamnopyranoside (5), quercetin 3-O-alpha-rhamnopyranoside (6), (-)-epigallocatechin (7), gallic acid (8), N-trans-caffeoyltyramine (9), and N-trans-feruloyltyramine (10). All of them were examined for their inhibitory effects on herpes simplex virus type-1 (HSV-1) replication in Vero cells. Both compounds 1 and 2 exhibited potent inhibitory activities in HSV-1 replication. Comparison of the IC50 values indicated that compounds 1 and 2 had higher inhibitory activities than the positive control acyclovir (38.6 +/- 2.6 vs. 55.4 +/- 5.3 microM, P < 0.001; 11.4 +/- 0.9 vs. 55.4 +/- 5.3 microM, P < 0.0005). Cytotoxicity was unlikely involved because no cell deaths were observable in the Vero cells following 5 day treatments with compound 1 or 2.