Two samples of polymer‐coated cobalt nanoparticles were synthesized and dispersed in agarose gel and water. The relaxivities r₁ and r₂ of the two samples were obtained at different temperatures (25, 37 and 40°C) and magnetic field strengths (1.5 and 3 T). The average cobalt core diameters of the two samples were 3.3 and 3.9 nm (measured by transmission electron microscopy); the corresponding average total diameters (cobalt core + polymer coating) were 13 and 28 nm (measured by dynamic light scattering). The larger particles had the higher r₁ relaxivity, whilst r₂ was similar for the two samples. There was no significant change in r₁ or r₂ relaxivities with temperature but r₁ at 1.5 T was approximately double the value at 3 T. The highest relaxivities were obtained at 1.5 T with values for r₁ and r₂ of 7.4 and 88 mM⁻¹ s⁻¹, respectively. These values are similar to those reported for iron oxide with larger core size, suggesting the potential of the cobalt nanoparticles for development and future use as a negative contrast agent. Copyright © 2008 John Wiley & Sons, Ltd.