Measurement of reticulo-endothelial system (RES) phagocytic function by clearance of intravenous micro-aggregated human albumin (HMAA) showed prolonged clearance in both patients (p < 0.001) and rats (p < 0.001) with extrahepatic biliary tract obstruction compared with non-jaundiced controls. After the administration of the immune stimulator, N-acetyl-L-α-aminobutyryl-D-isoglutamine, the mean HMAA clearance rate in jaundiced animals was similar to that of non-jaundiced controls. The implications of modifying RES phagocytic function to prevent overspill of endotoxins from the portal to systemic circulation in obstructive jaundice are discussed