Homologous or heterologous erythrocytes have been widely used for the estimation of intravascular volume of the liver. However, cross-species blood mediates immune response in the organ, and foreign cells are rapidly cleared from the plasma, indicating that heterologous erythrocytes may not be a suitable marker for determination of vascular space. This aspect was investigated in the perfused I-at liver preparation following bolus administration of human (heterologous) erythrocytes into the portal vein. To compare the extent of its distribution within the liver, rat (homologous) erythrocytes and Evans blue were chosen as the reference vascular and extracellular markers, respectively. Hepatic distribution of human erythrocytes was influenced by the perfusion medium (with and without protein) and injection number (first and second injections). Mean transit time and hence volume of distribution decreased in the presence of protein and repetition of the injection. Even in the presence of protein, the volume of distribution obtained for human erythrocytes was larger than that of the extracellular volume of the liver obtained with Evans blue (0.22 +/- 0.01 vs. 0.20 +/- 0.02 ml/g), indicating that they are not suitable for determination of intravascular volume of the perfused rat liver preparation. (c) 2006 Prous Science. All rights reserved.