A new procedure for assaying the phagocytosis of rod outer segments (ROS) by cultured rat pigment epithelial (PE) cells has been developed. Using an ROS antiserum and a double immunofluorescent labeling procedure, ROS attached to the external surfaces of these cells can be distinguished from those that have already been ingested. We have used this procedure to study the phagocytosis of ROS by PE cells isolated from normal rats and rats with inherited retinal dystrophy (RCS rats). With this approach we have been able to show that the attachment of ROS to the external surfaces of dystrophic PE cells does take place to a normal extent. However, only a small number of these bound ROS are subsequently ingested, demonstrating that the ingestion phase of phagocytosis is defective. After a 4-hr incubation during which ROS are continuously present, normal rat PE cells ingest about 80% of the ROS that have bound to the cell surfaces. In contrast, after this time period, less than 20% of the ROS bound to the dystrophic PE cells have been ingested. These results, as well as the results of pulse-chase experiments in which ROS are rinsed away after two hours and the incubation continued without further addition of ROS, have demonstrated that normal PE cells rapidly ingest most of the bound ROS, whereas the dystrophic PE cells show no such rapid ingestion. Both cell types, however, are able to slowly ingest additional bound ROS with time.