The authors investigated whether iron oxide particles can be used as a magnetic resonance imaging (MRI) contrast agent to image abscesses in a two-stage experimental design. Human buffy coat was incubated with iron oxide particles of different sizes and coatings. Smears of the incubation mixture were made on a glass slide and stained for iron. The percentage of iron oxide uptake was determined by counting 100 neutrophils and monocytes and scoring the number of cells that contain iron. Subcutaneous abscesses were created in the flanks of 18 Sprague-Dawley rats by injecting them with 0.1 mL of turpentine. Iron oxide was given intravenously, and the animals were imaged by MRI (1.5 T) 12 to 24 hours later. Different iron oxide coatings and doses were compared. The four different types of coating (constant fragment [Fc] of IgG, bovine serum albumin [BSA], lipid [Ferrosome], and dextran) had an uptake of 72% +/- 5.3%, 61% +/- 6.2%, 30.5% +/- 6.8%, and 5% +/- 2.5%, respectively. Comparison of two particle sizes (mean, 90 versus 35 nm) showed the large particles to have higher uptake (61% +/- 6.2%) compared with the small particles (6% +/- 1.8%) (P less than .001). Post-contrast imaging of the rats showed a hypointense ring around the abscess only in the animals injected with the lipid-coated agent. The effect was discernible within 12 hours after contrast injection and at a dose of 25 mumols iron/kg. Histologic sections showed phagocytic cells with iron granules in the periphery of the abscess. No hypointense ring on MRI or iron granules on histologic sections was seen around the abscess of the control animals or those injected with BSA-iron oxide or Fc-iron oxide. Lipid-coated iron oxide particles can be used to image abscesses by virtue of their phagocytosis into surrounding inflammatory cells. Positive uptake of these particles by human phagocytes in vitro suggests that similar results may be applicable in humans.