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There is rapidly increasing interest in the use of MRI to track cell migration in intact animals. Currently, cell labeling is usually accomplished by endocytosis of nanometer‐sized, dextran‐coated iron oxide particles. The limitations of using nanometer‐sized particles, however, are that millions of particles are required to achieve sufficient contrast, the label can be diluted beyond observability by cell division, and the label is biodegradable. These problems make it difficult to label cells other than macrophages in vivo, and to conduct long‐term engraftment studies. It was recently demonstrated that micron‐sized iron oxide particles (MPIOs) can be taken up by a number of cell types. In this study we examined the MRI properties of single MPIOs with sizes of 0.96, 1.63, 2.79, 4.50, and 5.80 μm. Furthermore, the capacity of cells to endocytose these MPIOs was investigated, and the MRI properties of the labeled cells at 7.0 …