A study is presented in which the inhibiting effects of eACA, PAMBA, AMCA and AMBOCA on fibrinolysis induced by three activators have been compared by a clot lysis method. The order of inhibitory potency was AMBOCA greater than AMCA greater than PAMBA greater than EACA. The activity ratios obtained were found to be influenced by the level of inhibition. At low levels of inhibition the differences between the inhibitory activities are usually more pronounced than at high levels. The degree of inhibition is dependent on the fibrinolytic activator used. The order of sensitivity to the inhibitors was, within the analytical conditions used, found to be: tissue activator greater than streptokinase greater than urokinase. The results are discussed in comparison to the different activity ratios reported by others.