Plasma exchange can induce changes in the biological systems of coagulation, fibrinolysis, complement and kinins, either by contact activation or plasma substitution. In order to know which of the two is responsible, the actual initial and final values and theoretical calculated final values were compared. Fibrinogen, Antithrombin III and platelets fell more than expected, and there was a significant increase in Platelet-Factor four and Betathromboglobulin which was greater than would produced by plasma substitution alone. Fibrinolysis is shortened and white cells rise. Complement and kinins were not significantly changed. During plasma exchange, contact activation actually triggers coagulo-fibrinolytic pathway and stresses cellular components.