An adaptation of our regular BCA Protein Assay, the Micro BCA Protein Assay Kit has been optimized for use with dilute protein samples, 2-40 ug/ml. The assay 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. 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.