Electron-microscope immunocytochemistry and antibody staining of nitrocellulose replicas of SDS gels (Western blots) were used in a developmental study to detect the presence and localization of opsin in the developing photoreceptors of rds (020/A) mutant mice and their BALB/c normal controls. Western blot analysis of isolated retinal membranes first detected opsin at 10 postnatal days in both strains. Opsin levels rose progressively with development in BALB/c normal retinas. In contrast, levels in the rds retina became undetectable by 30 days after peaking at 15 days. Specific binding of anti-opsin antibodies was first observed by immunocytochemistry at postnatal 5 days in the distal plasma membrane of the connecting cilium in both BALB/c and rds retinas. Thereafter, labeling intensity increased progressively with development in the BALB/c retina. Anti-opsin labeling remained localized primarily to the plasma membrane of the distal cilium and to the outer segment with the exception that light labeling of the inner-segment plasma membrane was observed from 5-15 postnatal days. Antibody binding to photoreceptors in the rds mouse retina predominated in the plasma membrane of the connecting cilium at 5 postnatal days, but opsin was present at higher density in the inner segment plasma membrane at 5-, 10-, 15- and 20 postnatal days, when compared with BALB/c photoreceptors. From 10-20 postnatal days opsin-rich vesicles were observed in the ventricular (subretinal) space of the rds retina. Maximum intensity of labeling was observed at 15 postnatal days. By 30 postnatal days, labeling of the ciliary and inner-segment plasma membrane decreased to near background levels.