The metallofullerenes, Hox C82 (x = 1,2) and Gd C 82 were generated, sublimed from carbon-arc soot, and further purified by HPLC. Sample purity was then judged by LD-TOF MS and compared with purities obtained by ICP-AE and NAA (Ho only) metal assays. Results indicated that the mass spectrometry data overestimated the purity of the metallofullerene samples by about a factor of two. The water-soluble metallofullerol derivatives, Hox C82(OH)y and Gd C82(OH) y were then prepared, and studies were conducted to determine their proton relaxivities in aqueous solution and magnetic properties in the solid state. The relaxivity of Gd C82(OH)y (R1 = 19.9 mM-1 sec.-1 at 20 MHz, 40°C) in aqueous solution was 5 times greater than commercially available MRI contrast agents, indicating that Gd C82(OH)y (6.4 muB at 100K) holds great promise as an MRI contrast agent. In comparison, Hox C82(OH)y did not enhance proton relaxation and had a magnetic moment lower than its Hox C82 precursor (Hox C82(OH)y = 3.8--5.2 muB at 300K, vs. Hox C82 = 5.5--7.8 mu B at 300K). Finally, full biodistribution studies of a water-soluble radioactive metallofullerene compound were conducted using 166Ho x C82(OH)y in BALB/c mice. For this work, the metallofullerol was neutron-activated (165Ho (n,gamma) → 166Ho) to prepare the 166Hox C82(OH) y radiotracer [Egamma(166Ho) = 80.5 keV] and was monitored after intravenous administration for up to 48 hours by dissection radioanalysis for comparison with a Na2[166Ho(DTPA)(H2O)] control study. Results showed 166Ho localization in the liver with slow clearance, as well as slow uptake by bone without clearance. In contrast, excretion of the control compound was nearly quantitative after 1 hour. The fate of 166Ho was also explored by a metabolism study of 166Hox C82(OH)y in Fischer rats. Results demonstrated 20% clearance of the 166Ho within 5 days. Together, these studies comprise a demonstration of metallofullerene materials utility in radiotherapy, radiodiagnostics, and MRI contrast agent applications.