To identify the hematogenous origin of macrophages in the photocoagulated retina, a technique of labeling blood monocytes with colloidal carbon prior to the coagulation was used. Colloidal carbon was given intravenously to adult pigmented rabbits twice daily for 5 days, and 3 days after the last injection xenon flash photocoagulation was performed on the retina. The circulating blood cells and the coagulated portions of the retina and choroid were then examined using electron microscopy. At the time of photocoagulation, circulating blood monocytes contained several clusters of carbon granules in the cytoplasm. Two days after coagulation, monocytes with carbon granules were observed to penetrate the Bruch's membrane. In the coagulated retina, macrophages with carbon granules were observed, while retinal pigment epithelial cells contained no carbon granules. Two months after coagulation, macrophages with carbon granules remained in the retina, and a few proliferated pigment epithelial cells contained carbon granules in the cytoplasm. These carbon granules were considered to be derived from degenerated macrophages. These results indicate that hematogenous monocytes invade the retina from the choroid through the Bruch's membrane and are transformed into macrophages, some of which degenerate in the retina and are phagocytized by pigment epithelial cells.