In chicken cells infected by Sindbis virus and exposed to a variety of membrane-active compounds, virus release was inhibited. In infected cells exposed to antiserum directed against the virion glycoproteins E1 or E2, retinol, cortisone, Pb++ or insulin, the processing of two Sindbis virus precursor polypeptides which lead to the formation of virion polypeptides was inhibited. The B-protein, which is the precursor to both envelope proteins, accumulated in cells treated by these compounds. This precursor is generally not detected in chicken cells, presumably because it is processed rapidly. The cleavage of the precursor PE2 to the envelope glycoprotein E2 was also inhibited. E2 was also absent in cells exposed to menadione or to antiserum prepared against uninfected chicken cells. Since each of the compounds tested interfered with Sindbis virus polypeptide cleavage, despite their diverse mechanisms of action, it suggests that perturbation of normal membrane fluidity can interfere with Sindbis virus budding.