Antibodies and fragments thereof have high affinity for human α-synuclein protofibrils and low binding of α-synuclein monomers, wherein the antibodies or fragments have specified Complementarity Determining Region (CDR) sequences. Compositions comprise such an antibody or fragment and methods of detecting α-synuclein protofibrils use such an antibody or fragment. In further embodiments, methods of preventing, delaying onset of or treating a neurodegenerative disorder with α-synuclein pathology comprise administering such an antibody or fragment, and such an antibody or fragment is used in the manufacture of a pharmaceutical composition for treatment of a neurodegenerative disorder with α-synuclein pathology. Such an antibody or fragment is used in the diagnosis or monitoring of the development of a neurodegenerative disorder with α-synuclein pathology, and in methods for reducing or inhibiting α-synuclein aggregation by administration of such an antibody or fragment.