Extraction of nattokinase, a fibrinolytic enzyme in natto bacillus, was attempted by the following 4 methods: 1) extraction with saline, 2) treatment with the organic solvents acetone, toluene and hexane prior to extraction, 3) alkaline treatment at pH 11.0 and, 4) autolysis in the presence of 0.1% NaN3 at 4\\xa1 C. Each fraction showed not only a strong fibrinolytic activity, but also H-D-Val-Leu-Lys-pNA and Suc-Ala-Ala-Pro-Phe-pNA amidolytic activities. Doses of 50–200 mg/kg natto bacillus (11011 active cells/g) were orally administered for rat experimental pulmonary thrombosis and to healthy human volunteers. A decrease in thrombus count and plasma euglobulin lysis time (ELT), as well as an increase in tissue plasminogen activator (t-PA), indicate that natto bacillus serves to activate plasma fibrinolysis in vivo.