The function of the reticuloendothelial system (RES) was elucidated in mice subjected to either a sublethal thermal or a mechanical trauma by using 51Cr labelled human red blood cells (51Cr-RBC) as a test substance. After i.v. injection at different times after trauma the fate of the test substance was investigated in three ways: (1) disappearance from blood, (2) uptake in liver,lungs and spleen and (3) urinary excretion of 51Cr. The ability of the RES to engulf the 51Cr-RBC was significantly reduced during the first posttraumatic days, but normalized gradually within 6 days. At 16 days after trauma the disappearance rate was significantly incressed in traumatized animals compared to controls. Organ uptake of 51Cr-RBC was markedly delayed after both types of trauma. Urinary excretion of 51Cr was normal after thermal but reduced after mechanical trauma. The significance of these results is discussed with respect to the "multi-factorial" influence exerted upon the RES by trauma. Forskarrapport