Tyrosine nitration is a covalent posttranslational protein modification derived from the reaction of proteins with nitrating agents. Protein nitration appears to be a selective process since not all tyrosine residues in proteins or all proteins are nitrated in vivo. To investigate factors that may determine the biological selectivity of protein tyrosine nitration, we developed an in vitro model consisting of three proteins with similar size but different three-dimensional structure and tyrosine content. Exposure of ribonuclease A to putative in vivo nitrating agents revealed preferential nitration of tyrosine residue Y115. Tyrosine residue Y23 and to a lesser extent residue Y20 were preferentially nitrated in lysozyme, whereas tyrosine Y102 was the only residue modified by nitration in phospholipase A2. Tyrosine Y115 was the residue modified by nitration after exposure of ribonuclease A to different nitrating agents: chemically synthesized peroxynitrite, nitric oxide, and superoxide generated by SIN-1 or myeloperoxidase (MPO)/H2O2 plus nitrite (NO2/-) in the presence of bicarbonate/CO2. The nature of the nitrating agent determined in part the protein that would be predominantly modified by nitration in a mixture of all three proteins. Ribonuclease A was preferentially nitrated upon exposure to MPO/H2O2/NO2/-, whereas phospholipase A2 was the primary target for nitration upon exposure to peroxynitrite. The data also suggest that the exposure of the aromatic ring to the surface of the protein, the location of the tyrosine on a loop structure, and its association with a neighboring negative charge are some of the factors determining the selectivity of tyrosine nitration in proteins.