Abstract C5b-6 complex was generated by activation of C7-depleted serum and purified 1200-fold based on its hemolytic activity. A homogeneous bimolecular complex with an s-rate of 11.5S and a m.w. of 328,000 daltons was obtained. The electrophoretic mobility was -3.4 × 10-5 cm2/volt sec. The subunit composition as revealed by SDS-polyacrylamide electrophoresis without reduction showed two protein bands: C5b (180,000) and C6 (120,000); and with reduction three subunits: C6 (128,000), C5bα (122,000), and C5β (75,000). Immunochemical analysis confirmed complex formation between C5b and C6 and showed the presence of one of the two neoantigens expressed by the SC5b-9 complex. Sixty nanograms of C5b-6 represented 108 hemolytically effective molecules. A hemolytic efficiency of 12 to 20 C5b-6 cell-bound complexes per lytic event was calculated from the uptake of radiolabeled C5b-6. The hemolytic efficiency of C5b-6 is dependent on the cell concentration. C5b-6-initiated lysis is strongly inhibited by serum low density lipoproteins with an inhibition constant of Ki = 34 µg/ml and by the S-protein with Ki = 39 µg/ml. Lipoproteins and S-protein form a stable, hemolytically inactive complex with nascent C5b-7. The mechanism of C5b-7 inhibition is interpreted as a competition reaction between the target cell membrane and the inhibitors for the transitory binding site in nascent C5b-7.