Vaginal application of human herpes simplex virus (HSV) antiserum, complement-inactivated antiserum, or IgG purified from antiserum protected mice (P < .001, P < .001, and P < .01, respectively) from visible signs of genital HSV-2 infection after subsequent vaginal inoculation with HSV-2 (10 ID50). Vaginal application of an anti-HSV-2 monoclonal antibody (MAb III-174) also protected mice against infection. This MAb, a neutralizing mouse IgG2A against glycoprotein D, prevented infection as determined by viral shedding from the vagina (P < .05), blocked 50% of visible signs of genital herpes infection at a vaginal dose of approximately 10 ng, and blocked 100% of visible signs of infection at a vaginal dose of 1 microgram (P < .001). These results suggest that vaginal applications of anti-HSV antibodies may help prevent sexual transmission of genital herpes infection.