In these studies, mice were simultaneously depleted of CD4 and CD8 T lymphocytes (T cell-depleted) by weekly intraperitoneal injections of rat monoclonal antibodies specific for L3T4 and Lyt-2 respectively, beginning 1 day before topical corneal infection with KOS herpes simplex virus type 1 (HSV-1). Control mice were mock-depleted by weekly injections of saline. All mice developed dendritic epithelial lesions 2-3 days after infection, which healed within 2 days. Fifty percent of the mock-depleted mice developed severe HSV-1 stromal disease, which began 9-14 days after infection. No skin lesions were observed in mock-depleted mice. In contrast, none of the T cell-depleted mice developed HSV-1 corneal stromal disease through an initial 30 day observation period. These mice did develop severe periocular skin lesions. After 30 days, the T cell-depleted mice were subdivided into two groups. In one group, T cell depletion was continued and the mice remained largely free of stromal disease for an additional 30 days (one of 30 mice developed a mild stromal haze). T cell depletion was discontinued in the second group. During the subsequent 30 days the CD4 and CD8 T cells in their lymph nodes and spleens recovered to approximately 50% of normal, and 43% (13 of 30) of the mice developed severe HSV-1 stromal disease. The skin lesions healed in all T cell-depleted mice between days 30 and 60, even when T cell depletion was maintained. Our findings demonstrate that T cells are both protective (preventing the spread of HSV-1 in the skin) and detrimental (inducing the destruction of the corneal stroma). Viral antigens seem to persist for at least 30 days in an apparently normal cornea but induce disease only when the cornea is infiltrated by HSV-1 reactive T lymphocytes.