The control of proteolytic activity in tissues is primarily under the influence of plasma inhibitors which function to rapidly inactivate specific target proteinases by either covalent interactions or trapping reactions. Each inhibitor is designed to control a specific proteolytic event, although many may show weaker activities against other proteinases. Kinetic experiments, however, indicate that reactions with target enzymes are much more rapid than with other and that modification of the inhibitor dramatically interferes with the rate at which inhibition occurs. This latter effect is almost certainly due to the fact that the inhibitor itself acts as a perfect substrate for the proteinase in question, resulting in a rapid association with the enzyme followed by a very slow dissociation so that the enzyme is essentially trapped in a complex.