Using a panel of monoclonal antibodies to guinea pig leukocyte antigens we have examined the phenotype of inflammatory cells present in focal chorio-retinal lesions developing in guinea pigs during S-antigen induced experimental autoimmune uveitis (EAU). The earliest choroidal infiltrates of T cells appeared to be T helper (Th) cells but in choroidal foci of inflammatory cells and in early retinal lesions, where rod outer segment lysis had occurred, the presence of a Pan T negative, T cytotoxic/suppressor (T c/s) positive population of effector cells was identified. In areas of more extensive retinal damage, the ratio of putative Th to T c/s cells was 1.1. The observation that B cell numbers increased progressively to become the predominant cell type in end state lesions, supports the concept that these may be the major suppressor cell population in guinea pigs. Infiltrating inflammatory cells and capillary endothelial cells express Class I and Class II antigen, however retinal pigment epithelium and Mullers cells did not express Class I or Class II antigens.