Thrombosis is one of the most important problems in the recent medicine. The mechanism of thrombolysis is not clearly elucidated. Using immunofluorescent technique, we examined the mechanism of ...thrombolysis in vitro in the present study. Blood clots were made of 4ml normal human blood at 37°C incubation for 24 hours. They were immersed either in Urokinase (UK) or saline solution. The degree of clot lysis was represented by decrease of each clot weight. Artificial thrombi were made by the method of Chandler. The thrombi were rotated at 37°C for two hours in the loop containing UK or saline solution. The effect of UK on thrombolysis was assessed by decreased weight of the thrombi. Clots and thrombi were observed by immunofluo-rescent technique with anti-human fibrinogen, anti-human plasminogen and anti human-urokinase rabbit serum. Results obtained were as follows. (1) Intensive fluorescence of plasminogen was detected generally along the fibrin network in the blood clots and in the red tail of the artificial thrombi. In the white head of the thrombi, on the other hand, faint fluorescence was found along the fibrin nets surrounding the platelet aggregate. (2) UK solution induced an enhanced lysis of clots and thrombi. (3) Immunofluorescence of Urokinase was observed along the fibrin network, especially at the margin or slit-like spaces of the thrombi. It was assumed that both of plasminogen and UK were found along the fibrin network and the activated plasmin in the thrombi played an important role in the thrombolysis, and that the red tail of the artificial thrombi which contained plasminogen more than in the white head was easily lysed.
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