Herpetic stromal keratitis (SK), a frequent cause of visual impairment, is considered to represent an immune-mediated inflammatory response to persistent herpes simplex virus virions or subcomponents within the corneal stroma. The experimental disease in mice involves the essential participation of T lymphocytes, but the role of T-lymphocyte subsets in either mediating or controlling the disease is uncertain. In this report, rat monoclonal antibodies were used to selectively deplete mice in vivo of CD4+ (helper-inducer) and CD8+ (cytotoxic-suppressor) T-cell populations and the effect on herpetic SK was evaluated. As measured by flow cytometry, mice treated with anti-CD4 monoclonal antibody (GK 1.5) were greater than 95% depleted of CD4+ T lymphocytes and mice treated with anti-CD8 monoclonal antibody (2.43) were 90% depleted of CD8+ T lymphocytes. Depleted and nonspecific mouse ascites-treated control mice were infected topically on the corneas with herpes simplex virus type 1, and the induction of various immune parameters during the acute infection was evaluated. CD4+-depleted mice failed to produce either a significant antiviral antibody or delayed-type hypersensitivity response but were capable of producing normal cytotoxic T-lymphocyte responses. In contrast, CD8+-depleted mice produced antiviral antibody and delayed-type hypersensitivity responses comparable with those in control animals, but cytotoxic T-lymphocyte responses were markedly reduced. Clinical observations of the corneas revealed that SK in CD4+-depleted mice was significantly reduced, whereas in CD8+-depleted mice SK developed more rapidly, was more severe, and involved a greater percentage of mice. These observations implicate the CD4+ T-lymphocyte subset as the principal mediators of SK and CD8+ T lymphocytes as possible regulators that control the severity of SK.