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ABSTRACT: A metal organic polyhedron (MOP) with four paramagnetic Fe(III) centers was studied as a magnetic reso-nance imaging (MRI) probe. The MOP was characterized in solution by using EPR and UV-vis spectroscopies, FT-ICR mass spectrometry, and in the solid state with single crystal x-ray diffraction. Water proton T1 relaxation properties were examined in solution and showed significant enhancement in the presence of human serum albumin (HSA). The r1 relaxivities in the absence and presence of HSA were 8.7 mM-1s-1 and 21 mM-1s-1, respectively, per molecule (2.2 and 5.3 per Fe) at 4.7 T, 37 °C. In vivo studies of the iron MOP show strong contrast enhancement of blood pool even at a low dose of 0.025 mmol/kg with prolonged residence in vasculature and clearance through the intestinal tract of mice. The MOP binds strongly to serum albumin and shows comparable accumulation in a murine tumor model as a covalently linked Gd-HSA contrast agent.