During the past two years I have made systematic examinations of the blood in patients suffering from various forms of diseases of the blood, hemorrhagic and thrombotic, with special reference to the variations in the amount of antithrombin and prothrombin. The most significant results have been obtained in cases of hemophilia and in so-called idiopathic or spontaneous thrombosis. The chief object of the present paper is to report the findings in these two latter conditions.METHODSThe specimens of blood used for examination were obtained in all cases from one of the superficial veins of the arm by means of a graduated syringe (Luer form). The syringe was sterilized previously with boiling water and before inserting the needle into the vein it was filled with sterile salt solution (sodium chlorid 0.9 per cent. by partially filling the syringe and then expelling the solution so as to leave