Abstract C′5 was isolated by isoelectric precipitation, Pevikon electrophoresis and chromatography on DEAE cellulose and hydroxyapatite. The sedimentation coefficient was 7.9 S for purified C′5 and for C′5 in serum. Molecular weight of 200,000 was estimated on Sephadex G 200. On reaction with EAC′4,2a,3, some C′5 was fixed (ca. 1/10), and the remainder was inactivated. In accord with this, at low ionic strength, 10 molecules were required for lysing one erythrocyte. At high ionic strength, the reaction rate was lower. The efficiency of site formation was also lower, due to dissociation of cell-bound C′5. The dissociated C′5 either recombines with EAC′4,2a,3 or becomes inactivated, the former (site-to-site transfer of activated C′5) being favored at high SAC′4,2a,3 concentration. There are two mechanisms of C′5 dissociation. One, spontaneous dissociation, is favored at high ionic strength and occurs even at 0°C. The other, a consequence of decay-release of C′2a, is favored by elevated temperature and occurs even at low ionic strength.