Bone marrow and spleen samples were studied biochemically to elucidate the nature of the material stored in the large histiocytes present in beta-thalassemic patients. Only the protein-bound N-acetylneuraminic acid (NANA) content of the spleen of homozygous beta-thalassemic patients was significantly higher than that of controls. In the bone marrow, which contained a large number of thalassemic storage cells, the lipid-bound NANA and the glucocerebroside showed a moderate increase, while the amount of protein-bound NANA was much greater than that in controls. These results, in agreement with ultrastructural and histochemical observations, differentiate thalassemic storage cells from other storage cells found in diseases characterized by an increased destruction of blood cells and suggest that there may also be an impairment of the sialoglycoprotein metabolism of the red blood cells in homozygous beta-thalassemia.