PURPOSE: To obtain modified and immobilized blood filter paper perfectly prevented from the elution of hemoglobin, by contacting blood filter paper with a specific modifying and immobilizing reagent to modify and immobilize hemoglobin of the blood filter paper.

CONSTITUTION: Guthrie filter paper is impregnated with several droplets of blood to be dried in air and the impregnated one is put in a test tube while a modifying and immobilizing reagent is added thereto and the whole is treated at about a room temp. or about 37°C in a thermostatic water tank. The obtained blood filter paper disc is perfectly prevented from the elution of hemoglobin and generates no trouble to a reaction system. The reagent composition is 20W83: 3W32:2W48:3W38, pref., 20W83:3W16:2W48:7W40 in the ratio of ether : water : acetone : methanol. In addition, in the case of methylene chloride : water : acetone : methanol. In addition, in the case of methylene chloride : water : acetone : methanol, it is 25W83:3W20:2W30:2W45, pref., 50W83:3W10:2W30:2W30 in the ratio thereof.