Phlyctenular conjunctival biopsy specimens obtained from seven patients presenting with ulcerated limbal phlyctens were examined with routine histology and immunohistochemistry using a panel of monoclonal and polyclonal antibodies and immunoperoxidase techniques. Large numbers of mononuclear phagocytes, dendritic Langerhans cells and polymorphonuclear leukocytes and moderate numbers of T-lymphocytes were observed in the epithelium. This infiltrate was most intense at the basal epithelial layers which expressed HLA-DR antigens. The underlying stromal inflammatory infiltrate was organized as perivascular cuffs and a scattered subepithelial infiltrate, and consisted of many mononuclear cells and poly-morphonuclear leukocytes. Among the mononuclear cells, monocyte derived cells dominated and included monocytes macrophages and dendritic cells. T-lymphocytes were present in moderate numbers, whereas B-lymphocytes and plasma cells, mostly of IgA class, were infrequent. These findings provided in situ immunohistochemical evidence that phlyctenular eye disease is the result of delayed type hypersensitivity immune response in which monocytes and monocyte derived cells play a central role.