In the developing photoreceptor cells of the homozygous rds mutant mice S-antigen is localized over the ciliary protrusion as in the control mice, and to a lesser extent over the inner segments, perikaryal cytoplasma and the cell terminals. As the outer segments develop in the normal retina, the discs become increasingly immunoreactive. In the rds/rds retina the outer segments fail to develop but small membrane bound vesicles, immunoreactive for S-antigen are extruded and phagocytized by the retinal pigment epithelium. In the retina of older mutant mice, as the photoreceptor cells degenerate slowly, the surviving cells continue to show persistent immunoreactivity for S-antigen in the different regions of the photoreceptor cells. In the heterozygotes the outer segments are reduced and appear abnormal, but the localization of S-antigen is similar to normal. In the receptor region of the normal retina and in the deviant membranous structures in the mutant retina the localization of S-antigen is similar to that of opsin. However, some differences in the subcellular localization of these two photoreceptor specific proteins have been observed. It is concluded that the rds gene acts subsequent to the synthesis of these proteins and possibly at the site of disc assembly.