The biodistribution and elimination of magnetic starch microspheres (MSM) were studied qualitatively and quantitatively be radioiron tracer studies and relaxation time measurements. One hour after injection of MSM (1 mg/kg of Fe), 85% .+-. 5% of the dose was accumulated in the liver and 6.5% .+-. 3.1% in the spleen. The hepatic clearance led to 50% reduction in the T2 relaxation time of liver tissue. This T2 effect was halved after 24 hours and T2 reversed to baseline value within 5 days after injection. The radioiron was gradually cleared from the liver with a t1/2 of 4 to 5 days. Six weeks after injection of MSM, 72% .+-. 7% of the radioiron dose was detected in the circulation in a nonsuperparamagnetic form associated with the erythrocytes. The results indicate a redistribution of iron from the liver and spleen via the erythroid bone marrow to the erythrocytes after injection of MSM.