Negatively charged ultrafine silica particles (average diameter 20 nm) were used as support materials for adsorption immobilization of porcine trypsin, horseradish peroxidase, and bovine catalase under various conditions, and the changes in the enzyme activities and the circular dichroism (CD) spectra of these enzymes upon adsorption were measured. Since the light scattering intensity of the ultrafine particles was very low, the activities and the CD spectra of the enzymes adsorbed on the particle surfaces could be measured. The enzymes adsorbed at pH around and above their isoelectric points (pI) showed high activities. On the other hand, the enzymes adsorbed at pHs below their pI had significantly diminished activities and showed large CD spectral changes upon adsorption. The extent of CD spectral changes in the enzymes upon adsorption correlated very closely with that of the activity reduction. Therefore, the conformational changes in enzymes upon adsorption are one of the important factors that reduce the activities of adsorbed enzymes. These results demonstrate that the ultrafine particles are not only a novel support for enzyme immobilization but also are helpful for the molecular understanding of the immobilized enzymes. dish and catalase from bovine liver were obtained from Wako Pure Chemical Industries. Since the purity of purchased enzymes was high according to sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, they were used without further purification. The relative molecular masses of trypsin, peroxidase and catalase are 23300, 40200, 232000, respectively, and their isoelectric points (pI) are 10.8, 7.2, 5.5, respectively. All other chemicals used were of reagent grade.