Fc receptors for immunoglobulin G in the liver sinusoidal wall were studied in the normal rat and in humans by applying peroxidase-antiperoxidase immunoglobulin G complexes to the frozen sections. Fc receptors were found to exist continuously along the sinusoidal lining. The receptors showed no zonal distribution in the rat, and they were generally scarce near the central veins and portal areas in humans. To characterize the sinusoidal cells, carbon or latex was given intravenously and endogenous peroxidase was demonstrated for the rat, whereas factor VIII-related antigen and endogenous peroxidase were demonstrated for the humans. In the rat, Fc receptors were detected on Kupffer cells, which were characterized by an intense endogenous peroxidase activity and ingestion of latex or quantities of carbon. They were also detected on sinusoidal endothelial cells, which were characterized by undetectable peroxidase activity and no ingestion of latex nor of a small quantity of carbon. In humans, Fc receptors were also present on Kupffer cells as well as sinusoidal endothelial cells, as identified by endogenous peroxidase and factor VIII-related antigen, respectively.