Clathrin, a polypeptide of molecular weight (MW) 180,000, is the main constituent of the polygonal network that forms the coat of coated pits and vesicles; these vesicles play a part in intracellular transport between membranous organelles. This function involves specific recognition of target membranes as well as fusion and fission events that must be coordinated with the assembly, partial disassembly or reorganization of the clathrin coats. To understand these interactions on a molecular level, information about the structure of clathrin and the interactions of clathrin with itself and other proteins is required. Here we show that purified clathrin coats dissociate reversibly into triskelions, structures composed of three usually bent, rather flexible legs irradiated from a centre. We have determined the molecular weight of these triskelions and conclude that they contain trimers of clathrin together with about three light molecular weight polypeptide chains.