Discussion and Summary: A systematic study of the Zymosan procedure for the purification of human properdin has been performed and optimum conditions for maximal yield and purity of properdin at each step of the method have been defined. The results of this investigation have been incorporated into a revised procedure for the purification of properdin which has permitted preparation of human properdin from serum in 45–80% yield with a purity of 900–1150 units of properdin/mg of nitrogen. Such fractions were purified about 2500-fold over serum and contained about 1 μg of nitrogen/unit of properdin. The stability of purified properdin to heat depended upon the initial concentration of properdin. Complete inactivation of solutions containing 100–200 units of properdin/ml occurred during heating at 100°C for 30 min, whereas complete inactivation of solutions containing 50 units and 5 units/ml occurred during heating at 66°C for 30 min and 56°C for 30 min, respectively. Purified properdin could be maintained for at least 8 months without appreciable loss of activity by sterile filtration and storage at 2°C or by lyophilization and storage under vacuum. The data presented in this report on the effect of physicochemical environment on the binding of properdin to Zymosan also provided additional information on the kinetic requirements of this interaction in serum. A detailed study of this reaction and of the inactivation of the third component of human complement by the properdin-Zymosan complex will be reported elsewhere. The biochemical, immunologic, and physicochemical characterization of purified properdin is presented in the following paper.