In retinas of rats (RCS) with inherited retinal dystrophy, cells interpreted as macrophages infiltrate the outer nuclear layer and subsequently appear in the interphotoreceptor space, where they accummulate during the course of the disease. The morphology and distribution of these cells and their relations to the pigment epithelial cells were investigated. Macrophages, regardless of their location, possessed morphological features that distinguished them from the pigment epithelial cells. Premelanosomes and melanosomes, typical of pigment epithelial cells, were never observed in macrophages. There was no evidence to indicate that, during the period studied, the pigment epithelial cells had become dedifferentiated or had migrated from Bruch's membrane. Macrophages, like pigment epithelial cells, phagocytized little or no outer segment material. The findings indicate that, at least during the interval studied, the cells that infiltrate the retina and interphotoreceptor space are macrophages rather than pigment epithelial cells.