To investigate the role of T cell subsets in the development of herpetic stromal keratitis (HSK) in a well defined model, we used an adoptive transfer approach in which thymectomized and T cell-depleted mice [T (-)] were reconstituted with different numbers of syngeneic immune T lymphocytes after topical corneal challenge with RE strain of herpes simplex virus-1. In vitro stimulated or unstimulated immune T cells obtained from cervical and retropharyngeal lymph nodes of mice with HSK were used in adoptive transfer experiments. Although T (-) mice developed an initial epithelial inflammation, stromal keratitis did not occur. Reconstitution experiments revealed that mice that received 2 x 10 (7) or more unfractionated immune T cells could develop HSK lesions with severity comparable to immuno-competent control mice. In mice receiving CD8 (+)-depleted populations, even fewer cells (5 x 10 (6)/mouse) were able to induce significant HSK. In contrast, mice that received similar or increased numbers of cells depleted of CD4+ T lymphocytes did not develop HSK. Immune T lymphocytes transferred to mice that were mock infected on the cornea did not develop HSK, indicating that the immunopathogenic cells were virus specific and not merely reacting to autoantigens. Histopathologic examination of the diseased corneas demonstrated that the stromal inflammation in euthymic normal and T (-)-reconstituted mice was characterized by extensive polymorphonuclear leukocyte infiltration. Scattered lymphocytes, and occasional macrophages also were observed. These results provide further evidence that HSK represents an immunopathologic process mediated mainly by CD4+ T cells.