Summary Addition of concanavalin A to BHK cell monolayers infected with vesicular stomatitis virus prevented the formation of mature virus particles. In these cells the virus glycoprotein (G) was inserted into the plasma membrane and the protein that is in close association with the ribonucleic acid, protein N, was found in the cytoplasm. At times when cells infected in the absence of the lectin were liberating virus into the supernatant medium, the M or matrix protein was found in association with the plasma membrane of the lectin-treated cells. The removal of the lectin from the cells with α-methyl-d-glucoside 3 h after infection was followed by the immediate release of mature virus particles. The rate of virus release from these cells was the same as that from cells infected in the absence of the lectin. Addition of cycloheximide, an inhibitor of protein synthesis, immediately after α-methyl-d-glucoside treatment of the cells did not alter the rate of virus production, suggesting that the proteins required for virus synthesis were available in the lectin-treated cells and that virus assembly took place without further protein synthesis on removal of the lectin.