Misfolding and pathogenic aggregation of α-synuclein (αSyn) is a hallmark of familial and sporadic Parkinson's disease, but the physiological state of the protein in cells remains unsettled. We have further examined our hypothesis that endogenous αSyn can occur in normal cells as a metastable, helically folded tetramer, not solely as the unfolded monomer long thought to be its native form. At this meeting, we reviewed our recent approaches for trapping αSyn in intact cells via in vivo crosslinking, a 5-step purification of αSyn from normal human brain, and the generation of new monoclonal antibodies to αSyn that enable general and oligomer-selective ELISAs. Crosslinking in intact living cells confirmed that αSyn occurs in the cytosol of neurons and non-neural cells in substantial part as metastable tetramers and related oligomers, plus varying amounts of free monomers. The non-pathogenic homolog, β-synuclein, forms closely similar oligomeric assemblies, suggesting that the oligomers we observe for αSyn are also physiological. In contrast to other normal oligomeric proteins (e.g., DJ-1), αSyn tetramers dissociate rapidly to monomers upon conventional cell lysis but are retained partially as tetramers if cells are lysed at high protein concentrations ('molecular crowding'). Thus, αSyn exists natively as helical tetramers that are in dynamic equilibrium with unfolded monomers. The tetramers appear relatively resistant to aggregation, in contrast to monomers, which may give rise to fibrillar inclusions.