The study of magnetic effects on biological system was started in response to Dr. Nose's wish for the development of extracorporeal immunomodulation therapy. In the extracorporeal immunomodulation system, monocytes or lymphocytes in blood are stimulated by the interactions with immune-active materials. The concept of magnetic field application was introduced in order to enhance the cell activation. Therefore, the applicability of magnetic field was studied. This paper was concerned with in vitro study of magnetic field effects on macrophage/monocytes and lymphocytes activations. Time-varying magnetic field could enhance macrophages/monocytes and lymphocytes activations.