Vaginal gel formulations containing the naphthalene sulfonate polymer PRO 2000 are being developed as topical microbicides to protect against infection with sexually transmitted disease (STD) pathogens. A mouse model was used to determine whether PRO 2000 could protect against genital herpes in vivo. Animals received a single intravaginal application of 15 μL of a 10% PRO 2000 aqueous solution or a 4.0% or 0.5% PRO 2000 vaginal gel formulation 20 s prior to intravaginal challenge with 4.0 log10 pfu of herpes simplex virus type 2. Treatment with the 4.0% gel provided complete protection against infection; treatment with the 0.5% gel or 10% solution provided 81% and 80% protection, respectively. Furthermore, the 4% gel provided significant protection even when viral challenge was delayed until 60 min after treatment. This is the first report to show that PRO 2000 can protect against infection with an STD pathogen in vivo.