PLASMA PROTEINASE INHIBITORS 657 maximum that should be allowed for effective control of a particular enzyme. This is based on the fact that while IJ (IPI can inhibit cathepsin G with a tl/2 in this range, a second inhibitor, IXI-antichymotrypsin (IX I Achy) has an even faster tl/2 of 5 msec. Teleologically, our interpretation is that IXIPI simply Gannot inactivate cathepsin G rapidly enough to control the activity of this enzyme and that IXIAchy has thus been developed for this purpose. Some of the reactions that have been measured, such as those between CI-inhibitor (CI-Inh) and Clr or cIs, probably do not represent true in vivo rates, because the enzyme (s) is in a muIticomplex. This may also be true for kallikrein and Factor Xu. Alternatively, there may be other undiscovered factors in plasma or tissues that increase the rates of inhibition in the manner in which heparin activates antithrombin III (AT III). In any case, with these few exceptions all the enzymes listed can be controlled by inhibitors that interact with a tili of less than 100 msec. The data given in Table 1 also show that the original function (s) given to at least two of the inhibitors, IX IPI, and IX IAchy, do not represent their true role in controlling physiological processes involving proteolytic enzymes. In fact, it is probable that several plasma inhibitors specifically control the activity of individual proteinases, and that many currently used common names must be revised as their functions become clear. Potential functions for plasma inhibitors should, therefore, not be stated unless the physical defect associated with a known deficiency, kinetic data indicating rapid complexing of specific enzymes, and the partitioning of the enzyme in question among all plasma proteins are available and compatible. Where deficiency states are not known, both of the latter two sets of data still aid markedly in determining whether the inhibitor can inactivate the enzyme in question rapidly enough to be of physiological significance.