The localization of the S-antigen was studied by electron microscopy in the rabbit eye, under three conditions of adaptation: A) 24-hour dark adaptation, B) 24-hour dark adaptation and 1.5-hour light adaptation and C) light adaptation for 30 hours. The Fab' fraction of the IgG of rabbit against the swine S-antigen was labeled with horseradish peroxidase (HRP) and was used as the marking antibody. In Group A, the HRP reaction products indicating the S-antigen were found mainly in the disk and plasma membranes of the outer segments of the photoreceptor cells. The reaction products were also found diffusely in the cytoplasm of the inner segment with the exception of the mitochondria and nucleus. In Group B, the phagosomes were found surrounded by the multilayered microvilli of the retinal pigment epithelial (RPE) cells; the reaction products were scarcely found in the phagosomes but were seen in the microvilli. Some weak reaction products were also encountered in the cytoplasm of the RPE cells. In Group C, after 30-hour light adaptation, many phagosomes were found but they contained almost no reaction products. However, the microvilli of the RPE cells surrounding the phagosomes showed a fair amount of the reaction products. The cytoplasm of the RPE cells contained abundant reaction products, particularly in the area of the basal infoldings. The reaction products were also seen in the Bruch's membrane and the endothelial cells of the choriocapillaris. It was thought that the S-antigen is taken up by the RPE during phagocytosis of the shed outer segments and is transported to the choriocapillaris.