3-Nitrotyrosine (3-NT) is a biomarker of nitrogen free radical species modified proteins in systemic autoimmunogenic conditions. This ELISA kit uses Competitive-ELISA as the method. The microtiter plate provided in this kit has been pre-coated with 3-NT. During the reaction 3-NT in the sample or standard competes with a fixed amount of 3-NT on the solid phase supporter for sites on the Biotinylated Detection Antibody specific to 3-NT. Excess conjugate and unbound sample or standard are washed from the plate, and HRP-Streptavidin (SABC) is added to each microplate well and incubated. Then a TMB substrate solution is added to each well. The enzyme-substrate reaction is terminated by the addition of a sulphuric acid solution and the color change is measured spectrophotometrically at a wavelength of 450 nm. The concentration of 3-NT in the samples is then determined by comparing the O.D. of the samples to the standard curve.