The nitration of tyrosine residues in protein occurs through the action of reactive oxygen and nitrogen species and is considered a marker of oxidative stress under pathological conditions. The most active nitrating species so far identified is peroxynitrite, the product of the reaction between nitric oxide and superoxide anion. Previously, we have reported that in erythrocytes peroxynitrite irreversibly upregulates lyn, a tyrosine kinase of the src family. In this study we investigated the possible role of tyrosine nitration in the mechanism of lyn activation. We found that tyrosine containing peptides modelled either on the C-terminal tail of src kinases or corresponding to the first 15 amino acids of human erythrocyte band 3 were able to activate lyn when the tyrosine was substituted with 3-nitrotyrosine. The activity of nitrated peptides was shared with phosphorylated but not with unphosphorylated, chlorinated or scrambled peptides. Recombinant lyn src homology 2 (SH2) domain blocked the capacity of the band 3-derived nitrotyrosine peptide to activate lyn and we demonstrated that this peptide specifically binds the SH2 domain of lyn. We propose that nitropeptides may activate src kinases through the displacement of the phosphotyrosine in the tail from its binding site in the SH2 domain. These observations suggest a new mechanism of peroxynitrite-mediated signalling that may be correlated with the upregulation of tyrosine phosphorylation observed in several pathological conditions.