The clearance of five radioactively labeled test substances--polyvinylpyrrolidone, colloidal albumin, antimony sulfur colloid, endotoxin, and heparin--by the reticuloendothelial system was studied after i.v. injection of these substances into rats. The participation of parenchymal, Kupffer, and endothelial liver cells could be determined after isolation and purification of these cell classes. Only endotoxin was almost exclusively taken up by Kupffer cells. All other substances were also taken up by both endothelial and parenchymal cells. From the rate of uptake, expressed as the endocytic index, it appeared that all substances, with the exception of polyvinylpyrrolidone, were taken up by adsorptive endocytosis. Although the specific rate of uptake by parenchymal cells was always slower than that shown by both Kupffer and endothelial cells, the total contribution of parenchymal cells to the clearance of polyvinylpyrrolidone, antimony sulfur colloid, and heparin by the liver was even greater than that of the other two cell classes. The results demonstrate that nonphagocytosing parenchymal and endothelial cells make an important contribution to the clearance of the various test substances which, in addition, is highly dependent on the nature of the substance used. This necessitates a revision of present concepts of clearance by the reticuloendothelial system.