1. The retarding effect of plasma on the action of trypsin can be measured quantitatively.

2. The nature of the reaction involved in effecting the retardation has been subjected to an experimental study.

3. Evidence is presented which indicates that the equilibrium between the inhibitive agent and trypsin is reached practically instantaneously and is rapidly and completely reversible.

4. This equilibrium has been studied by experiments in which we have observed (1) the effect of adding increasing amounts of plasma to a constant amount of trypsin, (2) the effect of varying the amount of trypsin while the plasma was constant, (3) the effect of dilution on the trypsin-plasma mixture.

5. The results of these experiments are discussed and it is stated that they are in quantitative agreement with the law of mass action.

6. An equation was found which fits the curves for the experiments mentioned in (4). This equation was developed from the assumption that 1 molecule of trypsin combined with 1 molecule of inhibitor to form 1 molecule of trypsin-inhibitor compound. The agreement between the results calculated by this equation and the observed results is satisfactory. It is pointed out that the equation contains two arbitrary constants and the bearing this fact may have on the calculated results is discussed.

7. We conclude from the results of our study that we have adduced evidence which suggests the following statement regarding the so called "antitryptic" property of blood. The inhibitive agent and trypsin combine to form an inactive but dissociable compound. The reaction in equilibrium is expressed by the equation

Trypsin + inhibitor ⇌ trypsin-inhibitor

The conditions of equilibrium are apparently governed by the law of mass action. The behavior of the equilibrium is therefore similar to the behavior of other equilibria between different inhibitive agents and enzymes discussed in the paper.

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