A modified procedure for measuring human lymphocyte responses to retinal antigens was introduced and found to be highly sensitive. The main modifications are the use of round bottom culture wells, high antigen concentrations (up to 100μg/ml) and extended incubation time (up to 9 days). Using this procedure, the majority of patients with uveitis, as well as a large proportion of healthy donors were found to respond positively (S.I.±2.0) toward S-antigen and interphotoreceptor retinoid-binding protein. The responses to the retinal antigens were further enhanced by enriching the cultured lymphocytes for the helper/inducer subset. The responses to the retinal antigens were inferior to those against tuberculin (PPD), in particular in subjects who had been inoculated with BCG. The“secondary”responses to PPD were always higher in magnitude, were stimulated by markedly lower concentrations of antigen and were detected earlier in culture. The notion that the responses to retinal antigen in healthy donors are“primary”in nature was further supported by the findings that (a) lymphocytes from cord blood samples resembled adult lymphocytes in their response to S-Ag and (b) healthy donor lymphocytes which were prestimulatedin vitrowith S-Ag reacted to this protein by a specific“secondary”fashion. The possible involvement of lymphocytes with reactivity toward retinal antigens in pathogenic autoimmune processes in the eye is discussed.