Due to their unique properties, magnetic nanoparticles (NPs) have large potential to be used for diagnostic and therapeutic applications in biomedics, and in this thesis NPs were to be synthesized with the objective of using them for cell tracking – monitoring of the in vivo migration of injected cells in the body, and possibly immobilization of these injected cells, being made possible by internalization of NPs in the cells in vitro. In order for the NP dispersions to be successful for this application, they needed to be non-toxic to cells and stable in water at physiological pH. They also needed to possess certain magnetic properties, in order for them to enhance the contrast when used for MRI, acting as so-called contrast agents. Besides this, they needed to be taken up by cells in vitro to a high extent, while the cells remained healthy. The cell uptake was to be assessed with Prussian Blue, a substance that turns blue in the presence of iron, and therefore stainability of the particles with this substance was another requirement. Today, the contrast agents under clinical use still have a great potential for improvement, and the objective was for the synthesized NP dispersions to show advantages over a commercially available contrast agent. Three methods were tested to render NPs stable in water, of which one was proven successful, in terms of the NPs to be colloidally stable in water at physiological pH, as well as the dispersions to show no cytotoxicity and higher contrast enhancement for MRI than what did the commercial contrast agent. Also, a modification of the method was proposed to obtain NP dispersions that further enhance the contrast.