1. A study has been made of the equilibrium existing between trypsin and the substances formed in the digestion of proteins which inhibit its action.

2. This substance could not be obtained by the hydrolysis of the proteins by acid or alkali. It is dialyzable.

3. The equilibrium between this substance (inhibitor) and trypsin is found to agree with the equation,

trypsin + inhibitor ⇌ trypsin-inhibitor

The equilibrium is reached instantaneously and is independent of the substrate concentration. If it be further assumed that the rate of hydrolysis is proportional to the concentration of the free trypsin and that the equilibrium conforms to the law of mass action, it is possible to calculate the experimental results by the application of the law of mass action.

4. The equilibrium has been studied by varying (a) the concentration of the inhibiting substance, (b) the concentration of trypsin, (c) the concentration of gelatin, and (d) the concentration of trypsin and inhibitor (the relative concentration of the two remaining the same). In all cases the results agree quantitatively with those predicted by the law of mass action.

5. It was found that the percentage retarding effect of the inhibiting substance on the rate of hydrolysis is independent of the hydrogen ion concentration between pH 6.3 and 10.0.

6. The fact that the experimental results agree with the mechanism outlined under 3, is contrary to the assumption that any appreciable amount of trypsin is combined with the gelatin at any one time; i.e., the velocity of the hydrolysis must depend on the time required for such a compound to form rather than for it to decompose.

7. The experiments may be considered as experimental proof of the validity of Arrhenius' explanation of Schütz's rule as applied to trypsin digestion.

8. Inactivated trypsin does not enter into the equilibrium.