Introduction. Some animal viruses mature from their host cells by budding out from the plasma membrane. In this process the virus nucleoprotein becomes enclosed within a plasma membrane vesicle, which is released into the extracellular medium. The early work of Wecker (1957) showed that the phospholipids in the influenza virus membrane are derived from pre-existing cellular phospholipids as would be expected from their mode of assembly at the cell surface. However, studies of the origin of the protein in the virus membrane revealed that most virus proteins were specified by the virus (Pfefferkorn & Clifford, 1964; Holland & Kiehn, 1970). The mechanism of budding bears not only on virus assembly but also perhaps on the assembly of differentiated membrane vesicles in the animal cell (Franke & Kartenbeck, 1976). Very little, for example, is known at the molecular level as to how coated vesicles are formed during endocytosis (Goldstein et al. 1979), or how the membrane vesicles involved in intracellular transport of secretory proteins are assembled (Palade, 1975).