Analysis of viral glycoprotein expression on surfaces of monensin-treated cells using a fluorescence-activated cell sorter (FACS) demonstrated that the sodium ionophore completely inhibited the appearance of the vesicular stomatitis virus (VSV) G protein on (Madin-Darby canine kidney) MDCK cell surfaces. In contrast, the expression of the influenza virus hemagglutinin (HA) glycoprotein on the surfaces of MDCK cells was observed to occur at high levels, and the time course of its appearance was not altered by the ionophore. Viral protein synthesis was not inhibited by monensin in either VSV-or influenza virus-infected cells. However, the electrophoretic mobilities of viral glycoproteins were altered, and analysis of pronase-derived glycopeptides by gel filtration indicated that the addition of sialic acid residues to the VSV G protein was impaired in monensin-treated cells. Reduced incorporation of fucose and galactose into influenza virus HA was observed in the presence of the ionophore, but the incompletely processed HA protein was cleaved, transported to the cell surface, and incorporated into budding virus particles. In contrast to the differential effects of monensin on VSV and influenza virus replication previously observed in monolayer cultures of MDCK cells, yields of both viruses were found to be significantly reduced by high concentrations of monensin in suspension cultures, indicating that cellular architecture may play a role in determining the sensitivity of virus replication to the drug. Nigericin, an ionophore that facilitates transport of potassium ions across membranes, blocked the replication of both influenza virus and VSV in MDCK cell monolayers, indicating that the ion specificity of ionophores influences their effect on the replication of enveloped viruses.Cells infected with enveloped RNA viruses provide valuable systems for elucidating the pathways involved in subcellular transport of membrane glycoproteins (8,20,44). As a result of viral inhibition of host protein synthesis, only one or a few viral membrane glycoproteins are synthesized in infected cells. These viruses possess limited genetic capacity, and the glycosylation and transport of their membrane glycoproteins to the cell surface are probably carried out by the same systems used by the host cell for biogenesis of its own membrane glycoproteins.Monovalent ionophores such as monensin, which is reported to interfere with the translocation of secretory as well as most membrane glycoproteins, have been used to characterize the pathways of intracellular glycoprotein transport in eucaryotic cells (18,(37)(38)(39)(40)(41)43). In addition, the effects of monensin on IgM and H2 glycosylation in lymphoid cells and fibronectin glycosylation in cultured human fibroblasts, as well as its influence on oligosaccharide maturation of