From Sloan Kettering: · Nattokinase is an extracelluar enzyme secreted by the bacterium Bacillus subtilis, used to ferment boiled or steamed soybeans resulting in a preparation called nattō. Nattō has been consumed as food in Japan for several centuries. Nattokinase is also available as a supplement and is most known for its effects on prevention and treatment of clots and to improve blood circulation.