Publisher Summary This chapter discusses the membrane attack complex proteins, C5b-6, C7, C8, and C9, of human complements. The cytolytic activity of complement was the first recognized function of the complement system. The five plasma proteins participating in this reaction are C5, C6, C7, C8, and C9 that together form a complex of Mr 1–2 million, referred to as “the membrane attack complex” or “MAC.” The methodology for the isolation of the terminal complement proteins is based on plasma as the starting source for complement proteins. The seventh component of complement (C7) is thought to be related to the sixth component (C6) because of similar physical and chemical properties and because family studies on genetic polymorphisms of C6 and C7 indicate a close linkage between the loci for both genes. The C5b-6 complex was first recovered by the precipitation of euglobulin from acute-phase serum. Because of the difficulty in obtaining such sera and the presence of C7 in these sera, which leads to the formation of SC5b-9, it is better to use activated C7-deficient plasma or serum.