The mechanism of the inhibition of trypsin, plasmin, and chymotrypsin by serum was studied using fibrin tagged with radioactive iodine as a substrate.

Enzyme-inhibitor relationships were studied by: (a) varying the concentration of serum; (b) varying the concentration of enzyme; and (c) by diluting the enzyme-serum mixture. The results indicate that the inhibition of trypsin, plasmin, and chymotrypsin is a stoichiometric and irreversible reaction.

By using the Lineweaver-Burk graphical method of analysis it was demonstrated that the inhibition of trypsin and chymotrypsin is a non-competitive reaction. This finding supports the conclusion that inhibition by serum is an irreversible type of reaction.

The substrate was found to exert a retarding effect on the activity of plasmin. The possibility of a plasmin inhibitor in fibrinogen was suggested.

The suitability of the various procedures used in evaluating serum proteolytic inhibition was discussed.

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