Quantitative methods have been developed for the estimation of C'3 and properdin. With these methods, the kinetics of the over-all reaction between properdin, zymosan, and C'3 were investigated. The reaction followed first order kinetics, provided zymosan was in excess, and the serum not diluted beyond 1/4. The reaction rate was proportional to the temperature, and an activation energy of 20,500 calories/mole was found.

The distinction between total properdin and available serum properdin is discussed.

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