Thrombolytic therapy with Urokinase (UK) has often been successful but it is very difficult to determine the effective dosage of UK. It is reported that after UK administration, plasmin fibrinolytic activity was immediately inhibited by alpha 2-Plasmin inhibitor (alpha 2-PI). In this study, we used UK on patients with myoma to prevent the occurrence of thrombosis after operation and the initial decrease in alpha 2-PI activities following UK administration was investigated to determine the minimum effective dosage of UK required to suppress alpha 2-PI. An attempt was also made to measure UK activity in blood by means of chromogenic substrate S-2444, and in some cases by administering 60,000 I.U. UK, alpha 1-Antitrypsin (alpha 1-AT), alpha 2-Macroglobulin (alpha 2-M), Antithrombin III (AT III) and Plasminogen (Plg) were measured at the same time. The results were as follows: 1) By the drip infusion method. In all doses, alpha 2-PI and UK activity showed no remarkable change. 2) By the one shot method. a) A decrease in alpha 2-PI was observed following both 48,000 and 60,000 I.U. UK administration. It was noted that in the case of 48,000 I.U. UK, alpha 2-PI showed the lowest level, 60% of the pre-administration level. b) UK activity showed a gradual increase in the case of 60,000 I.U. UK only and large changes in the other cases. c) alpha 1-AT, alpha 2-M, AT III and Plg produced no remarkable changes. This indicated that the effective dosage of UK for suppressing alpha 2-PI was at least 48,000 I.U. UK with the one shot method, and alpha 2-PI is a reliable indicator of the effectiveness of UK therapy.