Aldehyde oxidase (AOX)-immobilized silica gel (AOX-SG) was developed as a means to remove formaldehyde in the gas phase by enzyme degradation. AOX from the formaldehyde-tolerant microorganism Paecillomyces variotii IRI017, which we newly isolated from soil, was utilized. Amino group-binding silica gel (AminoSG) was prepared by APTE (3-aminopropyl triethoxysilane) treatment. All types of AOX-immobilized silica gels were oven-dried at 50 degrees C to constant weight, and formaldehyde degradation by the dried AOX-SG or AOX-aminoSG was examined. Humidified air containing 0.4-0.5 ppm of formaldehyde was passed continuously through SG columns packed with one of four types silica gels: SG, AminoSG, AOX-SG, and AOX-aminoSG. Formaldehyde in test air was removed in the early period in all types of SG by adsorption. With SG and AminoSG, however, formaldehyde removal decreased sharply and become zero within 20 d because of saturation of the gel by formaldehyde. With AOX-SG and AOX-aminoSG, removal of more than 90% of formaldehyde continued for more than 90 d. These results suggest that formaldehyde in the test air was adsorbed by silica gels and degraded by immobilized AOX.