Intercellular spaces directly open to lymphatic vessels are present in the mesothelial layer of the peritoneum. The peritoneal cavity is the origin of two major lymphatic networks: the thoracic duct drains the retroperitoneal region and enters the left subclavian vein; the right lymphatic duct collects lymph from parasternal and intercostal-paravertebral lymph ducts and drains mainly through the diaphragm and, ultimately, into the right subclavian vein. In the former network, lymph passes through iliac, renal, splenic, posterior gastric, portal, superior mesenteric, and cisternal lymph nodes; in the latter network it passes through more distal (eg, parathymic) and left and right posterior mediastinal lymph nodes. 26, 27 The authors posited that there was not only USPIO present in abundant amounts in the lymphatic tracts due to these networks but also SPIO, which tended to be trapped in the liver with only slight distribution to the lymph nodes. Cultured macrophages absorbed SPIO more readily than USPIO. Under conditions where the same number of macrophages respond to iron oxide compounds containing the same amount of iron, larger particles are recognized more easily. This is consistent with the hypothesis that upon intravenous (IV) infusion, reticuloendothelial cells in the liver and spleen trap SPIO more readily than USPIO. This study had some limitations. First, the number of mice was too small for an accurate elucidation of the pharmacokinetics and prevented the exclusion of interindividual differences. Second, it could not be confirmed that the minute amounts of USPIO and SPIO used were injected accurately into the peritoneal cavity. In some instances, the agent might have been injected subcutaneously or into the bowel. Third, due to technical challenges, the pharmacokinetics of intravenously delivered USPIO or SPIO was not studied. However, considering the mechanisms by which USPIO reached the liver earlier than SPIO and the result that similar amounts of USPIO and SPIO were distributed throughout the lymph nodes, it is important to compare the differences of distribution between USPIO and SPIO via two different injection routes (IP and IV).