We sell three substrates for the measurement of proteases in general - Protazyme AK Tablets (T-PRAK), Protazyme OL Tablets (T-PROL) and Azo-Casein (S-AZCAS). Azo-Casein has a 5-fold greater sensitivity to similar products supplied by other companies, e.g. Sigma. However, the standard curves are "curved" for trypsin and Chymotrypsin. In looking at a range of enzymes, including trypsin and chymotrypsin, the ratio of activities on the two tablet substrates were much the same. We would recommend the Protazyme AK tablets. With Protazyme AK tablets the standard curve is linear, but the assay format is not quite as simple as it is for Azo-Casein.