Alpha-synuclein is a 140 amino acid protein that forms the major component of the abnormal filaments that make up the Lewy bodies and Lewy neurites of Parkinson's disease and dementia with Lewy bodies. It is also the major component of the filamentous glial cytoplasmic inclusions of multiple system atrophy. Here we have used recombinant alpha-synucleins and peptide competition to show that the monoclonal anti-alpha-synuclein antibody LB509 recognizes amino acids 115-122 of human alpha-synuclein. The antibody strongly labelled filaments extracted from multiple system atrophy brain, showing the presence of residues 115-122 of alpha-synuclein. LB509 failed to react with mouse, rat and zebra finch alpha-synuclein, because of amino acid differences with human alpha-synuclein. Since LB509 recognizes human but not rodent alpha-synuclein, it will be a useful reagent for the characterization of mouse lines transgenic for human alpha-synuclein.