1. 1. A 100-fold purification of the third component of human complement (C′3) has been achieved, employing Fraction III-1 (Cohn Method 9) as starting material. The final product was electrophoretically heterogeneous but was free of the activities of other components of complement, properdin, plasmin and C′1-esterase.2. 2. The purified fraction derived from Fraction III-1 was active in the properdin system in restoring the bactericidal and virus neutralizing properties of serum deficient in the third component (R3) and fortified with properdin.3. 3. Although the purified fraction derived from Fraction III-1 was highly active in restoring the hemolytic activity of R3, it was only slightly active in hemolyzing erythrocytes in the state EAhuC′1, 4, 2. The significance of this observation is discussed.4. 4. Preformed properdin-Zymosan complex failed to inactivate C′3 in Fraction III-1 or in any fraction derived from it. Partial purification of C′3 from normal human serum was then performed. It was possible to show that the inactivation of partially purified C′3 by properdin-Zymosan complex required a factor present in serum and certain serum fractions. This reaction proceeded independently of divalent cations. These observations are discussed in terms of previous studies on the properdin system and in relation to the complexity of the reactions in which C′3 is involved.
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