Experiments indicate that the action of trypsin can be controlled by raw serum. When sufficient trypsin is added to raw serum, the serum is digested. Large amounts of trypsin are required, and even with large amounts, the equilibrium between digested and undigested protein in the flask does not appear to reach as high a level as when trypsin acts on heated serum. When trypsin is in process of splitting heated serum the addition of sufficient raw serum immediately stops the action. Even small amounts of raw serum (2 cc. of raw: 48 cc. heated) have an appreciable effect on the height of the curve of digestion. These experiments suggest the possibility that serum exerts its inhibiting action on trypsin by means of a definite substance. However, more recent experiments, to be published later, do not substantiate this theory.