Development expression of S-antigen and its mRNA in human and rat fetal retina was studied by immunocytochemical and in situ hybridization techniques. Immunocytochemistry indicated that S-antigen was present after 4 months gestation in the fetal human retina. In the rat, S-antigen was detected in the retina only after birth. In situ hybridization studies indicated that the S-antigen mRNA was present at 13 weeks gestational age in the human and at 15 days in the rat embryo. S-antigen mRNA was expressed not only in the retina but also in ocular tissues of neural crest origin in the fetus.