PURPOSE: A separating material capable of separating monocytes from lymphocytes in a leukocyte component at the same time in high purity in high yield without damaging their functions, containing an acidic functional group of the magnesium salt type on a carrier.

CONSTITUTION: A separating agent for separating monocytes from lymphocytes in a leukocyte component, containing an acidic functional group [e.g., (-SO3)2Mg] of the magnesium salt type on a carrier (e.g., polystyrene). The separating agent is brought into contact with the leukocyte component in a solution containing ≥1g/dl protein, so that monocytes and granulocytes are separated from the lymphocyte, because the lymphocytes has larger negative charge of cell membrane than the monocytes and the granulocytes, the monocytes and the granulocytes have higher cell adhesion than the lymphocytes, and the magnesium ion at the end of the acidic functional group can exhibit synergistic effect on the strengthening of the adhesion. If the granulocytes are preremoved from the leukocyte component, the monocytes can be isolated simultaneously.