We describe a direct colorimetric assay for copper in serum and biological samples using 2,2'-bicinchoninic acid (BCA), a common reagent in most laboratories. BCA offers the advantage of being highly sensitive and specific for Cu(I) which rapidly forms an intense purple complex in the presence of BCA. The complex has peak absorbances at 562 and 354.5 nm with molar absorptivities of 7.7 x 10(3) liter mol-1 cm-1 and 4.6 x 10(4) liter mol-1 cm-1, respectively. Interference by other metal ions, pH, and detergents is minimal and the results correlate strongly with atomic absorption spectrophotometry.