Quantitative analysis based on the measurement of Prussianblue-stained areas revealed that iron-positive areas in the liver were larger in the mice injected with USPIO than those injected with SPIO at almost all time points except the last one (828 vs 0 µm 2 at 30 minutes, 1299 vs 297 µm 2 at 1 hour, 3647 vs 853 µm 2 at 3 hours, 2256 vs 1497 µm 2 at 12 hours, 1885 vs 1168 µm 2 at 24 hours, and 1948 vs 2483 µm 2 at 48 hours), as shown in Figures 1 and 2 . The difference was significant at 30 minutes, 1, and 3 hours (P , 0.05). USPIO was quickly distributed throughout the liver; its distribution increased until 3 hours postinjection and decreased thereafter. On the other hand, the distribution of SPIO occurred more slowly and increased over a longer period of time. Unlike USPIO, SPIO was not detected in the liver at 30 minutes. Iron-positive areas at the hepatic sinusoid corresponding to the area harboring Kupffer cells coincided with macrophage-positive F4/80-stained areas, as shown in Figure 3 .