BCA reagent is a solution used in the Bicinchoninic Acid (BCA) assay to measure protein concentration. It contains bicinchoninic acid, which reacts with copper ions in an alkaline environment to produce a purple-colored complex in the presence of proteins. The intensity of this color is directly proportional to the protein concentration and is typically measured at 562 nm using a spectrophotometer. The BCA assay is widely used for its high sensitivity and compatibility with various sample types.