α-Synuclein (αsyn) mutations, overexpression, misfolding, and aggregation are associated with Parkinson's disease. This protein has been intensively studied in neuronal systems. However, αsyn is also present in extracellular fluids, such as cerebrospinal fluid and blood plasma. Recent studies have attempted to quantify its levels and compare these in various extracellular fluids of control and Parkinson's disease subjects. Data from these studies have been difficult to interpret, suggesting that more sensitive, standardized, and well-characterized assays of larger cohorts are required. Here, we describe the development of a new ELISA specifically for quantifying αsyn in human plasma. An initial assay, using a commercial anti-αsyn monoclonal antibody (211; Santa Cruz Biotechnology, Santa Cruz, CA) and based on a published protocol, was adapted for use in human plasma. In addition, we have developed a novel αsyn-specific antibody for the assay that has very high sensitivity and signal:noise characteristics. Assays with either antibody showed high specificity for αsyn, and detected it in a variety of sample types, including plasma. These assays can now be employed on large cohorts of patients and control subjects to determine whether plasma levels are altered in disease. Although measuring extracellular αsyn levels may prove to be a useful biomarker of Parkinson's disease, it should also be a powerful tool for basic research aimed at understanding the normal and pathological physiology of αsyn secretion. © 2010 Wiley-Liss, Inc.