The invention provides monoclonal antibody 5C1 and related antibodies. The 5C1 antibody binds to an epitope within residues 118-126 of α-synuclein. The antibodies of the invention are useful, for example, for treating and/or diagnosing disorders associated with α-synuclein, particularly accumulation of α-synuclein deposits. Such disorders include Lewy body diseases, such as Parkinson's disease, Diffuse Lewy Body Disease (DLBD), Lewy body variant of Alzheimer's disease (LBV), Combined Alzheimer's and Parkinson disease, pure autonomic failure and multiple system atrophy (MSA).