Human hepatoma cells, infected by vesicular stomatitis virus, offer a good system to study simultaneously the intracellular localization of a well defined transmembrane glycoprotein (VSV-G), a secretory glycoprotein (transferrin), and a nonglycosylated secretory protein (albumin) . We used monospecific antibodies in combination with 5-and 8-nm colloidal gold particles complexed with protein A to immunolabel these proteins simultaneously in thin frozen sections of hepatoma cells. VSV-G, transferrin, and albumin are present in the same rough endoplasmic reticulum cisternae, the same Golgi compartments, and the same secretory vesicles. In the presence of the ionophore monensin intracellular transport is blocked at the trans cisternae of the Golgi complex, and VSV-G, transferrin, and albumin accumulate in dilated cisternae, which are apparently derived from the trans-Golgi elements. Glycoproteins, synthesized and secreted in the presence of monensin, are less acidic than those in control cultures . This is probably caused by a less efficient contact between the soluble secretory proteins and the membrane-bound glycosyltransferases that are present in the most monensinaffected (trans) Golgi cisternae .Secretory proteins and plasma membrane glycoproteins in eucaryotic cells are synthesized in the rough endoplasmic reticulum (ER),' and are transported in a series of membrane vesicles to the cell surface (1, 2). A number of lines ofevidence suggests that both classes of proteins migrate through the Golgi complex, but it is not known whether exactly the same intracellular pathway is employed. In favorable cases, a combination of cell fractionation and immunoelectron microscopic techniques were used to show the transient accumulation of proteins such as vesicular stomatitis virus glycoprotein (VSV-G), histocompatibility proteins, and predominant secreted proteins in the Golgi cisternae (2-4). Additionally, the high-mannose oligosaccharides added to nascent secretory and plasma membrane glycoproteins undergo the same intracellular modifications ; many of the modifying enzymes have been localized either to the rough ER or to the Golgi complex (5,6).I Abbreviations used in this paper: Endo-H, endo-ß-N-acetylglucosaminidase H; ER, endoplasmic reticulum, PBS, phosphate-buffered saline; VSV-G vesicular stomatitis virus glycoprotein.