Irinotecan hydrochloride shows much different responses in each patient, and it has severe adverse effects. Therefore, a sensitive marker for the side effect of irinotecan on immunotoxicity may be able to prevent the severe complications by the early detection. We have recently developed a method to assess the immunotoxicity by measuring the productivity of TNF-alpha from whole blood containing monocytes when stimulated by lipopolysaccharide. By using this method, the effects of continuous low-dose irinotecan therapy on immunotoxicity were assessed in 10 patients with advanced gastric or colon cancer. When compared this method with the others such as white blood cell count, lymphocyte blastoid transformation by phytohem agglutinin (PHA), and natural killer cell activity in terms of the sensitivity, immunotoxicity by this method was found earlier than the other methods. Because our original method is easy to perform and sensitive as compared to the conventional methods, it can be widely used as one of the laboratory tests useful for patients treated with immunosuppressive agents.