The aggregation of α-synuclein in dopaminergic neurons is a critical factor in the etiology of Parkinson's disease (PD). Oxidative and nitrative stress is also implicated in PD. We examined the effect of nitration on the propensity of α-synuclein to fibrillate in vitro. Fibril formation of α-synuclein was completely inhibited by nitration, due to the formation of stable soluble oligomers (apparently octamers). More importantly the presence of sub-stoichiometric concentrations of nitrated α-synuclein led to inhibition of fibrillation of non-modified α-synuclein. These observations suggest that nitration of soluble α-synuclein may be a protective factor in PD, rather than a causative one.