The populations of lymphocyte subsets in the conjunctiva were assessed in monkeys enterically immunized with different preparations ofChlamydia trachomatisprior to ocular challenge. Subsets were identified in immunohistochemical studies employing cross-reactive anti-human monoclonal and polyclonal reagents. Ocular challenge of orally immunized animals resulted in more equal numbers of T-helper and T suppressor/cytotoxic cells, compared to the higher proportion of TSseen in naive monkeys. TH:TS/CTLratios of 1.0-2.0 and 0.1-0.5 respectively were observed. Orally immunized monkeys also showed proportionately more IgA- and IgG-bearing cells and less IgM-bearing cells. B cells of each class were commonly seen immediately under the epithelial basement membrane. Although oral immunization induced a number of changes in the proportions and distribution of lymphocyte subsets which subsequently appeared in the conjunctiva, these changes were not correlated with disease outcome, except for the appearance of increased proportions of follicular IgA-bearing cells in partially protected monkeys.