Significantly elevated Thrombo-test values after natto intake were observed in patients undergoing anticoagulant therapy after valve replacement operations. Corroborative tests with humans and rabbits to check the observations showed natto to be strongly antagonistic to warfarin. The author suspected that Bacillus natto contained in natto might produce a large amount of vitamin K in the intestinal tracts after taken in, and assumed that the antagonism of natto to Warfarin was attributed to the synthesis of vitamin K by the bacterium in the organ. The author then determined vitamin K in plasma of healthy persons after intake of natto to indicate increases of serum vitamin K. In anticoagulant therapy it is important to maintain the TT value within a certain therapeutic range for a long period of time. Thus the author believes that patients under administration of Warfarin should be given special instructions in food.