Abstract The effect of two cationic ionophores, monensin and A23187, on the replication of Sindbis and vesicular stomatitis virus in cultures of BHK and chicken embryo fibroblast cells has been studied. Treating these cells with the ionophores at 10 −6, M 2 hr prior to infection at high multiplicities of infection did not affect viral protein synthesis but severely inhibited release of viral particles into the media. Glycoproteins of these viruses were synthesized in normal amounts but they did not appear on the surface of infected cells. Proteolytic cleavage of Sindbis virus glycoprotein PE2 to E2 was inhibited in ionophore-treated cells, although fatty acid attachment to PE2 proceeded normally. Fatty acid attachment to VSV G protein and the posttranslational removal of mannose residues from G oligosaccharides occurred in the drug-treated cells. These data indicate that these ionophores block the movement of viral glycoproteins from the Golgi apparatus to the cell surface membrane where budding and release of these two viruses occur.