BCA reagent is a solution used in the BCA (Bicinchoninic Acid) assay to determine protein concentration. It contains bicinchoninic acid, which reacts with copper ions in an alkaline environment, forming a purple color when proteins are present. The intensity of the color is directly proportional to the protein concentration and can be measured using a spectrophotometer, typically at 562 nm. This assay is widely used due to its sensitivity and compatibility with various sample types.