DAB-Am64-(1B4M-Gd)64 is a novel macromolecular liver MRI contrast agent consisting of a polypropylenimine diaminobutane (DAB) dendrimer conjugated with a bifunctional DTPA derivative complexing Gd(III) . The characteristics of DAB-Am64-(1B4M-Gd)64, including a rapid liver accumulation, have recently been reported. In this study, dynamic micro-MRI with DAB-Am64-(1B4MGd)64 was obtained in a liver metastasis-model in mice using colon carcinoma cells to evaluate visualization of micrometastatic tumors as compared to Gd-DTPA. The dynamic micro-MRI with DAB-Am64-(1B4M-Gd)64 was able to homogeneously enhance normal liver parenchyma and visualize micrometastatic tumors of 0.3 mm diameter in the liver of the mice with better contrast than with GdDTPA. In conclusion, DAB-Am64-(1B4M-Gd)64, a new liver MRI contrast agent, is potentially useful for diagnosis of liver micro-metastasis. The presence of hydrophobic groups on Gadolinium chelates can induce hepatocellular uptake and excretion into the bile ducts, gall bladder, and intestines, resulting in visualization of hyperintense/bright liver in a T1weighted image. This type of liver MRI contrast agents, containing Gadolinium and enhancing the signal of normal liver parenchyma, have actively been investigated for many years, and are currently in clinical use. The first Gadolinium-based agent of this class, [Gd(BOPTA)(H2O)] (MultiHance), has been approved in Europe. A related chelate, [Gd(EOB-DTPA)(H2O)] (Eovist), is currently in Phase III clinical trials. This agent is excreted to a greater extent via the liver, roughly 50% for [Gd(EOB-DTPA)(H2O)] versus 2-4% for [Gd(BOPTA)(H2O)] , resulting in significantly greater liver enhancement. DAB-Am64-(1B4M-Gd)64 was found to accumulate in the liver at approximately 50 % of the injected dose (%ID) within 15 min post-injection, and homogeneously enhanced the hepatic parenchyma [Kobayashi, Magn Reson Med 2001; 46: 795-802]. Thus, DAB-Am64-(1B4M-Gd)64 appears to be a comparable agent to [Gd(EOB-DTPA)(H2O)] . Herein, imaging of experimental liver micrometastases of LS174T tumors, which were not visualized by other imaging modalities in living animals, was studied using dynamic micro-MRI with DAB-Am64-(1B4M-Gd)64 to evaluate its imaging qualities as compared to Gd-DTPA. To the best of our knowledge, no study has been performed relating serial imaging of metastatic liver tumors in mice.