Treatment of herpes simplex virus (HSV)-infected hairless mice with a 2% phosphonoacetic acid (PAA) ointment prevented the appearance of virus-induced skin lesions and subsequent central nervous system (CNS) involvement. Treatment started 24 h after infection significantly reduced the intensity of the skin lesions and also prevented CNS involvement. After four to six applications of PAA ointment, a moderate skin erythemia developed, followed by scaling and complete healing 7 days after cessation of the treatment. Mice treated early after HSV infection had low or undetectable levels of virus-specific antibodies but were completely resistant to reinfection. Early treatment prevented the development of a latent ganglionic infection, but treatment initiated 24 h after infection could not prevent the establishment of the latent infection. PAA-treated and HSV-infected mice with nondetectable levels of antibodies did not develop, with a single exception, a latent ganglionic infection unpon reinfection. The cell-mediated immune response determined by levels of [14C]thymidine incorporation in Ficoll-Hypaque-purified spleen lymphocytes cultures was low in PAA-treated mice; it increased slightly after challenge infection but was strong in mice that proved to harbor a latent HSV infection in the ganglia.