α-Synuclein aggregates are a major pathogenetic component of Parkinson disease (PD), and naturally occuring antibodies for this protein have been investigated as a biomarker of PD. A new study used ELISA to compare levels of naturally occurding antibodies for α-synuclein in the serum and cerebrospinal fluid of patients with PD and controls. No differences were found, but the investigators reported high inter-assay variability in antibody measurements, which could have biased the results. The observation highlights the importance of established protocols and quality control to the investigation of biomakers for PD.