In this study, the in vitro antiviral properties of prodelphinidin B-2 3,3'-di- O-gallate (PB233'OG) isolated from the bark of Myrica rubra (Myricaceae) was investigated. Results showed that PB233'OG exhibited anti-herpes simplex virus type 2 (HSV-2) activity with IC (50) values of 5.3 +/- 0.1 and 0.4 +/- 0.04 microM for XTT and plaque reduction assays, respectively. The IC (50) value increased with increasing MOI (multiplicity of infection). PB233'OG did not show a cellular cytotoxic effect at concentrations that possessed antiviral activity. Mechanistic studies demonstrated that PB233'OG inhibited HSV-2 attachment to the Vero cell, interfered with the penetration of HSV-2 into the Vero cell, affected the late stage(s) of the HSV-2 infection cycle, and also reduced the viral infectivity at high concentrations. It is concluded that PB233'OG exhibits various modes of action in its anti-HSV-2 effects.