Commercially available amino-modified polystyrene particles of size range 100-1,000 nm were radioactively labeled with [14C]-formaldehyde. A study of the circulation time and body distribution of these particles was carried out in mice. The animals were sacrificed at 1-30 min after intravenous administration of the particles. The blood and organ (liver, spleen, lung) profiles of particles were determined by measuring their radioactivity by means of liquid scintillation counting. In general, larger particles were eliminated from blood faster than smaller particles. The blood elimination half-life ranged from 1.36 to 4.92 min. The particles were mainly taken up by the liver with larger particles being taken up faster than the smaller particles. At 30 min after injection, 60% of the administered 100 nm particles were present in the liver, whereas 85% of the 100 nm particles were found in the liver. Accumulation in the spleen was 1-3% of the total number administered in the entire size range. At 1 min after injection, less than 3% of the total dose administered was present in the lungs and this value decreased rapidly to less than 1% at 2 min. The only exception occurred for 100 nm, where 2.35% was present in the lungs at 2 min after injection.