Abstract In summary, the complement system represents a remarkably complex system of interacting proteins noteworthy for several features: 1. (1) The assembly and activation of complex proteolytic enzymes, each composed of more than one protein, which act sequentially on specific substrates. 2. (2) The biological activity of these enzymes requires the transfer of proteins from the fluid phase to the surface of target cells. This is possible, at least in part, by the ability of C3b and C4b to bind covalently to such surfaces. 3. (3) The system is extremely efficient; each of the proteolytically generated fragments appears to perform an important physiological function. 4. (4) The final event is the assembly of a macromolecular structure able to penetrate the cell membrane and in so doing lyse the cell.