1Heparin was found to have an inhibiting effect on various fibrinolytic activators by themselves, namely on urokinase, pig heart activator, stable tissue activators contained in prostatic extracts and myoma extracts, and on the labile bone marrow activator. The inhibiting effect of heparin monosulphuric acid was equal to that of heparin.2Heparin was also found to inhibit the fibrinolytic activity in urokinase‐ and streptokinase‐activated human plasma and the activator activity which develops in plasma after injection of nicotinic acid and after application of venous stasis.3Inhibition of fibrinolytic activators required by themselves large amounts of heparin (10–1000 units per ml). In the presence of plasma even low concentrations of heparin (0.5 to 2.5 units per ml) caused some inhibition. In none of the concentrations tested could any enhancing effect of heparin on fibrinolysis be noted.4Heparin had no effect on plasmin activity when plasmin was mixed with heparin alone or when heparin was added to plasma with plasmin activity.5With heparinized blood it was found necessary to add a larger amount of thrombin in the determination of the euglobulin clot lysis time if false short times were to be avoided.6The clinical significance of the antifibrinolytic effect of heparin is disc