The Kupffer cells, as fixed macrophages, belong to the mononuclear phagocyte system (MPS), the former reticulo-endothelial system (RES). Their ultrastructure has some characteristics, i.e. ergastroplasmic peroxidase activity and worm-like structures, which allow a very sharp identification. Some particular "bridge-like" arrangements of Kupffer cells in the capillary lumen seem to provide them with a special ability of "trapping" the passing white blood cells. The important endocytotic function is analysed from the viewpoint of ultrastructural features, and with particular reference to the phagocytosis of damaged red blood cells. Several drugs and particulate substances acting as Kupffer cells stimulants are presented. This stimulation reaction concerns woth increased number and enhanced endocytotic activity. The origin and kinetics of Kupffer cells are discussed, with reference to own results which substantiate the theory of local proliferation and blood supply, the bone marrow being however the main source of hepatic macrophages. A few ultrastructural data on Kupffer cells involved in pathological processes are presented.