Colloidal suspensions of iron oxide and metal iron nanoparticles prepared by laser pyrolysis have been obtained by coating the particles with dextran in an aqueous media giving rise to biocompatible ferrofluids. The structural characteristics of the powders and the size of the particles and the aggregates in the colloidal suspensions have been analysed and correlated with the magnetic properties of both solids and fluids. For the first time, to our knowledge, a stable ferrofluid based on metal particles (<10 nm) has been obtained with aggregate sizes of  nm. In comparison to iron oxide based products, this material exhibits higher saturation magnetization (45 emu g⁻¹) and susceptibilities (4000 emu/g T). In addition, the nuclear magnetic resonance response of the ferrofluids has been measured in order to gain information about the influence of the crystallochemical and magnetic properties on their relaxation behaviour. The main parameter affected by the presence of the magnetic nanoparticles is the transversal relaxation time T₂ and the corresponding relaxivity R₂ value that is of the order of 400 (mmol/l)⁻¹ s⁻¹. It has been shown that R₂ value increases not only by using iron metal instead of iron oxide but also by increasing the crystal size of the particles. From this study an evaluation of the possibilities of these materials as contrast agents for magnetic resonance imaging has been made.