To study the role of serum lipoproteins in immune hemolysis, human serum was depleted of lipoproteins by centrifugation at a density of 1.21. The density was raised to this value by addition of NaBr. Treatment of serum with high concentrations of NaBr, KI or KSCN was found to inactivate C′3, C′4 and C′5. The same salts produced inactivation and changes in the physicochemical properties of purified C′3, C′4 and C′5. Lipoprotein-depleted serum was fully active in immune hemolysis after addition of purified C′3, C′4 and C′5. No lipid could be detected in the purified components. It is concluded that serum lipoproteins are not required for immune hemolysis by human complement.

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