To our knowledge, this is the first study comparing the biodynamics of intraperitoneally injected USPIO and SPIO. SPIO (Resovist) is the contrast agent used in MRI and generally its safety in the clinical setting has already been established. USPIO used in this study is different in particle size from that of SPIO, but very similar in composition. Ferumoxtran-10, similar to the USPIO used in this study, was well tolerated in phase I and II studies in adult humans. 20, 21 In addition, in a safety study conducted in adult rabbits, no inflammation or fibrosis occurred after intraperitoneal (IP) injection of NC100150, a USPIO similar to ferumoxtran-10. 22 IP irondextran supplementation is known to be effective and safe in correcting iron deficiency in rats and humans. 23, 24 In our study, IP injection was safe and never caused the death of any mouse. USPIO was distributed to the liver as early as 30 minutes postinjection; it peaked at 3 hours and decreased thereafter. SPIO appeared later and its distribution throughout the liver increased over time. At 12 hours postinjection, the hepatic distribution of both agents was comparable. A greater amount of USPIO than SPIO tended to be distributed to the lung parenchyma around the airway, possibly because USPIO was presented more easily to the small lymphatic vessels around the airway. There was no significant difference in lymph node accumulation between the two groups of mice.