An efficient method for therapeutic treatment of a leukemic patient is disclosed in which the patient's body fluid under external circulation is brought into direct contact with an adsorbent material capable of adsorbing the leukemic cells in the body fluid specifically and selectively. The leukemic cell-adsorbent material, which is used by filling a column to form an adsorbent bed, is a conjugate of a lectin protein extracted from, e.g., Dolichos beans or soybeans coupled with a physiologically inert carrier material such as a polysaccharide in the form of beads or a superparamagnetic material in the form of iron oxide-based magnetic beads.