Summary: The conversion of EAC'1,4 to EAC'1,4,2 by purified C'2 has been investigated with respect to its kinetics and the principal factors influencing this reaction. A mathematical theory has been developed and its applicability evaluated. From kinetic evidence and on the basis of titration data, the transformation of EAC'1,4 to EAC'1,4,2 has been shown to involve a single reaction step, which means that interaction of one SA2C'1,4 site with C'2 converts a cell to the state EAC'1,4,2. It follows that hemolysis by antibody and complement is a one-hit or noncumulative process. On the basis of these results a method has been developed for measurement of C'2 in molecular terms. The limiting-component principle of Hegedus and Greiner has been shown to be invalid. The over-all “titer” of complement depends primarily on the supply of C'2, but the availability of C'1, C'4 and the C'3 factors also plays a role because their concentrations influence the efficiency with which C'2 is utilized.