Few years ago it was reported by Kurahasi from the Physiology Laboratory here, that the extract of the peanut and the soya-bean contained an anticoagulant substance, of which chemical nature and working mechanisms having been undecided. The author investigated on this subject and came to the following conclusions: Considering from the fact that 1) The anticoagulant action of the extract did not interfere with the presence of Ca-ion, 2) The extract acted as an antagonist to the action of serum thrombin, 3) The extract acted as an antagonist to cephalin-protein-compound, it may he concluded that the extract acted as antiprothrombin as well as as antithrombin.