To compare the phagocytosis of D-and DM-USPIO, we used the murine macrophage cell line, J774.1 (RIKEN Cell Bank; Wako, Saitama, Japan), which is widely used in research on macrophages. [29] [30] [31] Cells (8 × 10 5 ) were seeded in 12 multiwell cluster plates (Corning Inc., Corning, NY) and grown at 37°C for 24 hours in 1 mL growth medium (RPMI; Nacalai Tesque, Kyoto, Japan), supplemented with 10% bovine fetal calf serum by volume (Invitrogen, Carlsbad, CA) and 1% by weight mixed penicillin and streptomycin solution (Nacalai Tesque). The medium was then replaced with fresh medium. D-USPIO or DM-USPIO (10 µg iron oxide and 100 µL, respectively) were added, and the plates incubated for 1 hour for cell labeling. The medium was again replaced and the wells incubated for 24 hours. To identify intracellular iron oxide accumulations, the cells were stained with Prussian blue. The amount of iron contained in cell lysates was measured by atomic absorption photometry (AA-6800; Shimadzu, Kyoto, Japan). Cell labeling was performed as described in a previous study. 32