In the Royal College of Surgeons rat, the migration of phagocytic cells into the subretinal space accompanies photoreceptor cell death during the early stages of retinal dystrophy. These are followed closely by cellular alterations in the retinal pigment epithelium, Müller cells, and outer retinal vessels. We have identified the phagocytic cells as microglia and hypothesized that they may be involved in the above cellular changes. Thus, we developed procedures for their isolation and growth. Our study shows that retina-derived microglia (1) are positive for microglial markers Griffonia simplicifolia isolectin B4, Mac-1 alpha, phosphotyrosine, and vimentin; (2) are highly phagocytic; and (3) respond to macrophage colony stimulating factor by proliferating. This culture system would provide a valuable tool in studying mechanisms of cellular alterations in retinal disease.