Transport vesicles derived from the Golgi apparatus are thought to mediate biosynthetic transport across the Golgi stack. These vesicles are surrounded by a protein coat whose principal constituents are coatomer (a complex of seven distinct subunits or COPs) and ADP-ribosylation factor (ARF, an N-myristylated small GTP-binding protein). The coat proteins of the COP-coated vesicles were originally defined by ultrastructural criteria, however, and it is possible that important but minor coat proteins or cytoplasmic proteins needed for coat assembly may have been overlooked. Here we show that coatomer and ARF are the only cytoplasmic proteins needed for the assembly and budding of COP-coated vesicles. COP-coated buds may therefore form essentially by self-assembly from Golgi cisternae after an initial step in which GTP is used to allow ARF binding.