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Sequestration and degradation of red blood cells (RBC) are believed to occur in part in the liver, but the magnitude and cellular localization of this process remain uncertain. This problem was studied in rats by investigating isolated parenehymal and sinusoidal cell populations of the liver. After digesting the perfused liver with pronase, hepatic sinusoidal cells were isolated free of RBC and debris. Of the isolated cells, 90 % were phagocytic, as judged by their uptake of colloidal ~gSAu or of aggregated albumin-lalI administered in vivo After administration of spherocytic (heat-treated) RBC, however, only about one quarter of the isolated cells were found to contain phagocytized RBC. This apparently distinct popula~ tion of RBC-phagocydzing cells is designated as '°erythrophagocytic (EP)" ceils. The EP cell population was further characterized functionally by its specific phagocytosis of coltoidaI carbon and of 99mtechnetium-sulfur colloid and histochemically by its peroxidase activity. The role of the EP population in the catabolism of RBC-hemoglobin was studied in isolated hepatic sinusoidal ceils by assay of microsomal heme oxygenase (MHO), which is the inducible enzyme system that converts heine to bilirubin. The MHO activity of individual sinusoidal isolates was related directly to their content of EP ceils

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