Urokinase type plasminogen activator (u-PA) was purified from three different chest fluids obtained from patients with liver cirrhosis and pleuritis, aplastic anemia and pneumonia, and lung tumor, and the relationship between molecular weight and plasminogen activator (PA) activity was examined by zymography. The molecular weights of u-PAs from the chest fluids were 200 Kd, 150-180 Kd, 95 Kd, 55 Kd, 44 Kd, 33 Kd and 14 Kd, and PA activity was observed at molecular weights of 95 Kd, 55 Kd and 33 Kd. Fibrin binding of u-PA was observed at molecular weights of 55 Kd and 33 Kd.