Our BCA Protein Assay Kit is a detergent compatible formulation based on bicinchoninic acid (BCA) for the colorimetric estimation of total proteins. The reaction product of this assay is purple color, which is formed by the chelation of two molecules of BCA with one cuprous ion. This water-soluble complex exhibits a strong absorbance at 562 nm that is nearly linear with increasing protein concentrations over a broad working range of 20-2,000 ug/ml. The macromolecular structure of proteins, the number of peptide bonds and the presence of four particular amino acids: cysteine, cystine, tryptophan and tyrosine are reported to be responsible for color formation with BCA. Studies with di-, tri- and tetrapeptides suggest that the extent of color formation caused by more than the mere sum of individual color producing functional group. Accordingly, the protein concentrations are generally determined and reported with reference to standards of common protein, such as bovine serum albumin (BSA). A serial dilutions of known protein concentration are prepared and assayed alongside the unknown(s) before the concentration of each unknown is determined based on the standard curve. The assay is compatible with ionic and nonionic detergents, offers less protein-to-protein variation than dye-binding methods, and compatible with 96-well microplates.