We have identified a new coat protein in clathrin-coated vesicles from bovine brain by urea-SDS gel electrophoresis. The protein was purified from Tris-solubilized coat proteins either by combination of hydroxyapatite chromatography and gel filtration or more rapidly in a single step by immunoaffinity chromatography. The purified protein binds to clathrin triskelia and thereby promotes clathrin assembly into regular 50-100-nm cages. We propose for the new protein the name auxilin (Latin auxilium, meaning support). Auxilin migrates as a 110-kD polypeptide in standard type SDS-PAGE, but in the presence of 6 M urea shifts to a position corresponding to 126 kD. Gel filtration in 6 M guanidinium hydrochloride gives a molecular weight of approximately 86,000. The native protein is monomeric in 0.5 M Tris. Antigenic reactivity and two-dimensional peptide maps gave no evidence of gross similarities between auxilin and any of the other known coated vesicle-associated proteins. Since the structural organization of auxilin does not resemble that of the ubiquitous heterotetrameric HA1 and HA2 adaptor complexes, that are believed to connect clathrin to receptors, it is unlikely that it functions as an adaptor. Immunoblotting did not reveal the presence of auxilin in tissues other than brain. If auxilin and AP 180 are indeed both confined to neuronal cells, as the immunochemical evidence suggests, it might be inferred that both serve to adapt clathrin-coated vesicles to an as yet undisclosed function unique to this cell type.