Macrophages, one of the immunologically competent cells centrally contributing to host defense mechanisms, can be stimulated by a variety of immunomodulatory agents to exert antitumor activity. However, many problems, such as serious side effects attributed to the high-dosage regimens of the drugs, should be addressed for successful tumor immunotherapy using immunomodulatory agents. Thus, it is essential to develop carrier systems that use much lower doses of the drugs. This article reviews the carrier systems of drugs for effective immunopotentiation, especially macrophage activation of antitumor function. Since research in this field has just started, few investigations, including ours, have been reported.