The enhancement of guinea-pig complement by car☐ypeptidase A is due primarily to the effect of the enzyme on the activity of the fourth component (C4). The result is an increase in the number of SAC14 capable of reacting with terminal components to produce lysis without any detectable change in the ability of SAC14 to react with C2. as shown by the rate of generation of SAC142 and the amount of C2 consumed. Experiments utilizing guinea-pig C4 deficient scrum indicate that the enzyme acts on C4. either cell-bound or in the fluid phase, and not on an inhibitor associated with this component.