Plasma membrane vesicles are shed from monolayer cell cultures during incubation in low concentrations of formaldehyde and other sulphydryl blocking reagents. In both 3T3 and SV3T3 cells disulphide reducing agents, including dithiothreitol and mercaptoethanol, potentiate formaldehyde-induced vesiculation. Plasma membrane vesiculation is shown to be a temperature-dependent phenomenon which occurs optimally between 22 and 37 degrees C and at pH 7.0 to 7.5. Membrane shedding is an energy-dependent phenomenon, requiring monovalent and divalent cations and slightly hypertonic medium. Plasma membrane vesiculation is not affected by pretreatment of cells with inhibitors of protein synthesis, i.e. cycloheximide, nor by agents which disrupt the cytoskeleton.