The general assay for proteolytic activity using casein as a substrate was described in 1947. A modification using azocasein has been described, in which digestion by calpain results in colored peptides soluble in trichloroacetic acid (TCA), so that the intensity of the color in the supernatant from TCA precipitation is a function of proteolytic activity. The assay is simple, reliable, and sensitive, but not specific, although the absolute requirement of calpain for Ca2+ is used to impart some degree of specificity 1).