1. The inhibition by suramin of complement components, and of blood clotting, fibrinolytic, and plasma kinin forming factors depended on the conditions of the assay and on the substrates used.2. Haemolysis by complement was more effectively inhibited in red blood cell suspensions, than in agarose gel plates. In esterolytic tests, the activation of component 1 (C1) to C1 esterase was significantly inhibited by 0.1-0.3 mM suramin, and the activity of C1 esterase by 0.5 mM suramin.3. Part of the anticoagulant effect of suramin is due to inhibition of the action of thrombin on fibrinogen.4. Suramin did not inhibit fibrin degradation by the fibrinolytic system in plasma. In esterolytic tests, the activation of plasminogen was more potently inhibited than the activity of plasmin.5. Activation of plasma kallikrein, measured either by kinin formation or by esterolysis, was inhibited by 0.1-0.3 mM suramin. Active plasma kallikrein was inhibited by 0.3-0.5 mM suramin. Pancreatic kallikrein was weakly inhibited, and urinary kallikrein not at all.