The term ‘complement’ was originally used to describe an activity present in normal (non-immune) serum which was necessary to produce lysis of antibody coated cells. It is now recognized that complement is a system made up of a substantial number of protein components, many of which are now fairly fully characterized; and that it mediates a wide variety of the biologically important consequences of antigen–antibody interaction. Of these, immune haemolysis— although it remains the most widely studied model system—is certainly not the most important. Complement is a system, like the others that have been discussed, whose activation at the right time and place is presumably of adaptive importance; whose activation in other circumstances is the exact reverse. This type of system has recognizable characteristics: multiple factors, generally existing in inactive forms, and requiring sequential activation; and multiple homeostatic mechanisms for limiting activation. Complement with its multiple functions shows these features to a well marked degree and even out of the five systems discussed at this meeting, this is probably the most complex.