A new method for the isolation of C6 and C7 by affinity chromatography of human serum with anti-C6 and anti-C7 coupled to Sepharose is described. C6 and C7 prepared by this method are hemolytically fully active, homogeneous proteins obtained in 25% yield. A comparison of the properties of isolated C6 and C7 gave the following results: The amino acid composition of the two proteins is very similar. The m.w. calculated from the amino acid content is 124,800 for C6 and 120,800 for C7. Both components are single chain glycoproteins migrating upon electrophoresis at pH 8.6 as beta 2-globulins, Both proteins are polymorphic as detected by isoelectrofocusing in polyacrylamide gels and range in their isoelectric points from pH 6.15 to 6.7. The UV spectra reveal only minor differences; the extinction coefficients are: EC6 = 1.71 cm2 X mg-1 and EC7 = 1.92 cm2 X mg-1. CD-spectra show 8% alpha-helix and 10% beta-structure for C6 and 10% alpha-helix and 14% beta-structure for C7. The structural similarities of C6 and C7 suggest their evolution from a common ancestral gene.