A vaginal application of a commercially available contraceptive jelly containing nonoxynol-9 (N9) prevented vaginal transmission of herpes simplex virus type 2 (HSV-2) infections in the mouse. When N9 jelly was delivered to the vagina with the virus inoculum, 20 s before inoculum, or 5 min before inoculum, mice were completely protected from visible infection (P < .001). Protection lasted for at least 30 min (P < .03), and significant protection occurred even when the N9 jelly was delivered 15 min after the HSV-2 inoculum (P < .05). These results are consistent with results of studies using N9 products in other animal models and suggest that N9-based contraceptive products can provide significant protection against vaginal transmission of enveloped virus infections in animals.