The Bio-Rad Protein Assay is a dye-binding assay in which a differential color change of a dye occurs in response to various concentrations of protein. 1 The absorbance maximum for an acidic solution of Coomassie Brilliant Blue G-250 dye shifts from 465 nm to 595 nm when binding to protein occurs.2,3,4 The Coomassie blue dye binds to primarily basic and aromatic amino acid residues, especially arginine.5 Spector 6 found that the extinction coefficient of a dye-albumin complex solution was constant over a 10-fold concentration range. Thus, Beer’s law may be applied for accurate quantitation of protein by selecting an appropriate ratio of dye volume to sample concentration