Antithrombotic function of doenjang was investigated using thoroughly desalted aqueous extract. In the presence of the extract, thrombin suffered loss of its fibrin-clotting activity, whose extent increased in a dose-dependent manner. The active substance involved in thrombin inhibition showed a high thermal stability. Results of Sephadex G-25 permeation chromatography indicated that there were at least two soluble thrombin inhibitors in doenjang with different molecular sizes.