Prussian blue-stained histopathological sections showed marked iron uptake by macrophages embedded in atherosclerotic plaques in the aortic walls of rabbits injected with either type of USPIO. This was not the case in the control animals. Iron uptake was dose-dependent ( Figure 4 ). Immunohistochemical staining (RAM11) showed that the localization of iron deposits and of macrophages coincided ( Figure 5 ). Quantitative analysis of iron-positive areas in the aortic wall revealed that they were significantly larger in rabbits treated with dose 3 DM-than in those treated with dose 3 D-USPIO (P , 0.05). In addition, in different aortic sections, these areas were significantly larger in the rabbit treated with dose 1 or dose 2 DM-USPIO than in that treated with an equivalent dose of D-USPIO (P , 0.05) (Figure 6 ). At dose 3 of D-and DM-USPIO, the amount of iron accumulated per unit weight of aortic specimen was not different (Figure 7) .