The early distribution of intraperitoneally injected USPIO to the liver and the absence of significant differences in the lymph node distribution of USPIO and SPIO are primarily attributable to differences in the absorption route. The trapping of intravenously administered USPIO by reticuloendothelial cells in the liver and spleen has been reported to be difficult because the particle diameter is small and the particles persist in the blood. 11, 12 Lukas et al 25 who studied the absorption route of intraperitoneally administered compounds, suggested that particles absorbed by the subperitoneal capillary system reached the liver via the hepatic portal system. The earlier hepatic distribution of USPIO suggests that it is delivered via this pathway.