Lamellar keratoplasty specimens from a patient with Terrien's marginal degeneration and a patient with Mooren's ulcer were compared using routine histopathologic and immunohistochemical staining with an avidin-biotin-peroxidase complex. Less than 25% of the resident cells in the Terrien's marginal degeneration specimen expressed major histocompatibility class II antigens compared with 75% to 100% of the resident cells in the Mooren's ulcer specimen. The ratio of CD4 cells (T-helper/inducer) to CD8 cells (T-suppressor/cytotoxic cells) in the Terrien's marginal degeneration specimen was almost 1:1 compared with 24:1 in the Mooren's ulcer specimen. In addition, less than 5% of the infiltrating cells from the Terrien's marginal degeneration specimen stained positive for CD22 (B cells), compared with 25% to 50% from the Mooren's ulcer specimen. These data may help explain why Terrien's marginal degeneration runs a more benign course than does Mooren's ulcer and provides a rationale for the use of immunosuppressive drugs to treat Mooren's ulcer.