Magnetic resonance imaging (MRI) using contrast agents has been widely used to diagnose vascular diseases, to visualize the internal structure of organs, and to monitor transplanted cells and tissues. We recently developed a novel contrast agent for MRI using polysaccharide-modified magnetic iron oxide nanoparticles. The polysaccharide-magnetic particle complex has an advantage of low toxicity to cells and tissues and slow blood clearance over conventional contrast agents. However, because the surface charge of the particles is negative, transplanted cells such as pancreatic islet cells are less labeled. To overcome this problem, a complex of a polysaccharide and a positively charged magnetic metallic compound has been developed. These newly designed magnetic nanoparticles were efficiently transduced into various cells (for example pancreatic islet cells, MIN6 cells, HepG2 cells, hepatocytes, and somatic stem cells). The present chapter mainly describes the establishment of a cytotoxicity test system using our polysaccharide-based magnetic iron oxide nanoparticles. As a promising cell evaluation system, a new three-dimensional cell culture system "cell-array system" has been established. In the future, the system will be useful as a powerful tool for drug development.