The human hepatoma cell line, Hep G2, was used as a model system to delineate the morphological aspects of receptor-mediated endocytosis of asialoglycoproteins (ASGP). These hepatoma cells, which have several structural features in common with liver parenchymal cells, contain a well-differentiated endocytotic apparatus which includes coated vesicles and CURL (compartment of uncoupling receptor and ligand) similar to that found in rat liver cells in vivo (Geuze et al., 1983). In order to precisely identify the route of ligand from the cell surface to the CURL sorting vesicles, Hep G2 cells were allowed to endocytose in synchrony ASGP bound to 12 nm colloidal gold for 5 to 60 min. Thereafter the subcellular distribution of the gold particles was examined in ultrathin cryosections. This study demonstrates that, once internalized, ASGP ligand enters a system of CURL tubules prior to delivery to the lumenal contents of attached endocytotic vesicles. These vesicles then in turn transform into multivesicular bodies and secondary lysosomes. Thus, the tubular portions of CURL provide the entry for ASGP ligand into the sorting compartment.