Comparison has been made of measurements of trypsin activity by a substrate containing RIHSA (radioactive iodinated human serum albumin—Abbott Laboratories) and the commonly used methods of Anson and Kunitz. In this method, trypsin was incubated with an albumin substrate containing RIHSA, followed by precipitation of the undigested substrate with sulfosalicylic acid and measurement of radioactive digestion products in the supernatant fluid. Trypsin was found to be most stable in solution at a pH near 4.5 using the RIHSA method. Its activity was maximal at pH 6.5-7.0. The RIHSA method did not provide as sensitive a test for low concentrations of trypsin as did the methods of Anson and Kunitz. However, because of its simplicity for assay of tryptic activity, the RIHSA method is considered to be of definite value when enzyme concentrations to be assayed are in excess of 2000 T.U. per ml.