AbstractBlood contact with foreign surfaces induces a certain degree of activation of the defence systems including complement. Recent data using complement specific inhibitors have shown that complement is not only associated with, but in fact contributes to the whole body inflammatory reaction seen as a complication of cardiopulmonary bypass (artificial surfaces) and is responsible for the hyperacute rejection of xenografts (foreign endothelium). Complement activation products, particularly the terminal SC5b‐9 complex (TCC), is a sensitive indicator of bioincompatibility. Modifications of the artificial surfaces by coating with heparin improves both coagulation and complement compatibility. The use of specific complement inhibitors in general improves biocompatibility both of artificial materials and foreign endothelium.