A monoclonal antibody that binds a-synuclein, and that binds tau fibrils, and methods of using the monoclonal antibody, are provided. The monoclonal antibody may be hybridoma clone 2F11. Also provided is a composition comprising the monoclonal antibody 2F11 and a pharmaceutically acceptable carrier, adjuvant, vehicle or excipient. A method of reducing active a-synuclein in a subject in need thereof is also disclosed. The method involves administering an amount of the composition comprising 2F11 to the subject. The monoclonal antibody 2F11 may also be used to determine the a-synuclein, or tau fibril, level in a biological sample.
