To compare phagocytosis of USPIO and SPIO by cultured cells murine macrophage cell line J774.1 (RIKEN Cell Bank, Wako, Saitama, Japan) was used, which is widely used in research on macrophages. [16] [17] [18] Cells (5 × 10 4 ) were seeded in 12-well multiple well culture plates (Costar®; Corning Inc, Corning, NY) and grown at 37°C for 24 hours in 1 mL of growth medium (RPMI-1640; Nacalai Tesque, Inc, Kyoto, Japan) supplemented with 10% (v/v) fetal bovine serum (FBS) (Invitrogen Ltd, Carlsbad, CA) and 1% (w/v) penicillin and streptomycin solution (Nacalai Tesque, Inc). After 24 hours, the medium was replaced, USPIO or SPIO was added (10 μg : 100 μL) and the plates were incubated for 1 hour for cell labeling. After this, the medium was again replaced and the wells then incubated for 24 hours. To identify intracellular iron oxide accumulation, the cells were stained with Prussian blue. The amount of iron contained in cell lysates by atomic absorption photometry (AA-6800; Shimadzu Co, Kyoto, Japan) was measured. Cell labeling was as described in a previous study. 19 