Natto is a traditional Japanese food made from soybeans fermented by Bacillus subtilis natto. Natto and natto fungi contain many physiologically active substances, including dipicolinic acid, nattokinase, and vitamin K2. We used a rat hindleg perfusion model to show that the novel substances present in natto improve thrombolytic activity in the blood. The perfusion sample was extracted by mixing natto with twice the amount of water and heating at 121°C for 30 min. The effect of the extract on fibrin dissolution was measured using the rat hindleg perfusion test and the fibrin plate method. In blood vessels perfused with the natto extract made using the Miyagino strain, tissue-type plasminogen activator(t-PA) ac-tivity increased by 4.8 ± 2.1 times (n = 8). Five varieties of commercially available natto were examined. The average in-crease in fibrinolytic activity was 5.8 ± 3.0 times (n = 20).