Biologic nitration of protein tyrosine (to form 3-nitrotyrosine) is a recently described phenomenon that is associated with many diseases. We have devised a proteomic methodology to identify these modified proteins. This utilizes protein fractionation by two-dimensional polyacrylamide gel electrophoresis (2-D PAGE), partial transfer onto polyvinylidene difluoride (PVDF) membranes, and Western blot analysis using an antinitrotyrosine antibody to identify the proteins. Alignment of the Western blots with the partially transferred 2-D PAGE gels enables identification of immunopositive protein spots. These are then excised and trypsin digested. Proteins are then identified using either matrix-assisted laser desorption ionization-time of flight mass spectrometry or capillary liquid chromatography tandem electrospray mass spectrometry. Nonspecific crossreactivity of the antibodies is determined using reduction of protein bound 3-nitrotyrosine to 3-amino tyrosine using sodium dithionite.