In order to evaluate the potential of superparamagnetically labelled neutrophils as inflammation-specific contrast agents for magnetic resonance imaging (MRI), various types of magnetite particles were associated with isolated human neutrophils in vitro and subsequent effects on characteristic cellular functions investigated. Exposure to uncoated magnetite caused strong irreversible cell aggregation, whereas uptake of polystyrene-embedded magnetite microcrystals (Estapor M1-0.70/60) occurred with only minor changes in neutrophil adhesion properties, cellular metabolism, and chemotactic behaviour in vitro. As revealed by MR phantom imaging, Estapor-labelled neutrophils generated visible contrast above a threshold concentration of 1-2 micrograms Fe/g in a tissue-equivalent environment. Intravenous administration of labelled neutrophils to rabbits resulted in local magnetite accumulation up to 2.4 micrograms Fe/g tissue in artificial inflammation sites. Subsequent T2-weighted imaging of an intramuscular abscess clearly demonstrated the expected hypointense area surrounding the typically bright core of inflammation.