The mode of inhibition by alpha-1-antitrypsin of chymotrypsin, trypsin and pancreatic elastase was examined by a kinetic method. All three enzymes were completely bound to alpha-1-antitrypsin; therefore, the dissociation constant of the enzyme-inhibitor complex was too low to measure with these methods. The dissociation constants for the three enzyme-inhibitor complexes were estimated to be less than 5 - 10 minus 9 M. However, alpha-1-antitrypsin could be specifically displaced from Sepharose-bound elastase with an irreversible inhibitor of that enzyme. Additional experiments showed that dioxane, 20% (v/v), blocked 100% of the inhibition of elastase, 16% of the inhibition of trypsin, and 0% of the inhibition of chymotrypsin. The effect was reversed by diluting the dioxane to 1% (v/v). These findings indicated that alpha-1-antitrypsin was tightly bound to the three enzymes studied but did not allow discrimination as to the nature of the inhibition. However, the competitive mode of inhibition was suggested by the displacement of alpha-1-antitrypsin from elastase by an irreversible inhibitor of the enzyme that bound covalently at the enzyme-active site. The variable susceptibility of the enzyme to blockage of the inhibition by low concentrations of p-dioxane suggested that hydrophobic bonds may be important in the interaction with alpha-1-antitrypsin.