Magnetic nanoparticles were prepared by a reduction-precipitation method and coated with an amino silane coupling agent. Guanine (Gua) was conjugated to the magnetic nanoparticles (MNPs) using glutaraldehyde as a cross-linker. Common techniques (Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, and vibrating electron microscopy) were used to assess the properties of the particles. Structural investigations showed that amino silane-coated MNPs had a particle size of about 40-60 nm in diameter with a spherical morphology. The guanine-conjugated MNPs were radiolabeled with Tc-99m(I)-tricarbonyl core (Tc-99m(CO)(3)-MNP-Gua) with a labeling yield of 72 +/- 4 %. Pure radiolabeled magnetic particles were obtained by washing them with saline solution, and the radiochemical purity of Tc-99m(CO)(3)-MNP-Gua was 98 +/- 2 % in the final solution. The biologic distribution of guanine MNPs was assessed in New Zealand rabbits using a gamma camera. In the in vivo experiment, a high level of radioactivity was observed in the lungs and liver soon after intravenous administration of Tc-99m(CO)(3)-MNP-Gua.