Cayman's Protein Carbonyl Colorimetric Assay is a colorimetric assay for the measurement of oxidized proteins. The carbonyls of protein samples are derivatized using 2,4-dinitrophenylhydrazine. Formation of a Schiff base produces the corresponding hydrazone which can be analyzed spectrophotometrically at 360-385 nm. This assay can be used to measure oxidized protein in plasma, serum, cell lysates, and tissue homogenates.