Rice (Oryza sativa) seedlings were grown in the presence of 5 mM orthosilicic acid (H₄SiO₄, IUPAC name: tetrahydroxyl silane). More than 80% of silicon absorbed in the rice was localized in the alcoholinsoluble residues (AIR) fractions. AIR refers to cell wall materials in the text. Driselase (commercial cell wall hydrolyzing enzyme preparation) released 3% of silicon present in the rice AIR into water-soluble fractions. Size-exclusion high performance liquid chromatography/inductively coupled plasma atomic emission spectroscopy (HPLC/ICP-AES) showed that an aqueous silicon-containing substance with high molecular weight was present in the water-soluble fractions. The compounds were stable at pH 6.5, while commercial silica sols which were stable at pH 9.0 were insoluble at the neutral pH. The stability in neutral pH may be due to the presence of a complex of polysaccharide and protein in the molecule. These results imply that silica-containing macromolecules exist in rice cell walls.