Methods for detecting -synuclein oligomers in vivo comprise administering an antibody to an individual suspected of carrying soluble -synuclein, wherein the antibody is produced from a stabilized soluble -synuclein oligomer and is capable of binding the stabilized soluble -synuclein oligomer, wherein the stabilized soluble -synuclein oligomer has a lower formation rate to a non-soluble aggregated form than a non-stabilized soluble oligomer of the -synuclein, and wherein the antibody is labelled with a detectable marker; and detecting the presence of any complex formed between the antibody and soluble -synuclein by detection of the marker.