We studied the requirement for CD40⁺ cells in the resolution of vaginal infection with avirulent herpes simplex virus type I (HSV-1) in vivo using CD40-deficient mice, which were susceptible to infection with avirulent HSV-1. Compared with wild-type mice, CD40-deficient mice could not eliminate HSV-1 virus effectively from the vaginal mucosa and produced lower amounts of interleukin-12 and interferon-γ. These results show that the induction and activation of CD40⁺ cells are important for HSV prevention, facilitating the activation of T cells to induce an efficient HSV clearance from the vaginal mucosa and to prevent lethal illness due to HSV infection.