In lung specimens, a few USPIO and SPIO particles were observed early; they were located in the parenchyma around There was no significant accumulation in the heart, great vessels, kidneys, or gastrointestinal tract. As the controls harbored abundant stores of iron, visualized as Prussianblue-positive areas, macroscopically it was observed that there was almost no difference between them and USPIOor SPIO-treated mice with respect to the spleen. Control mice manifested no significant iron deposits in organs other than the spleen.