A futon (Japanese quilt) was beaten to disperse mite allergens into the air in a closed room, and the airborne allergens were collected both by Andersen air sampler for particle size analysis and by slit air sampler for kinetic analysis of the clearing of the allergens from the air. After extraction of the allergens from the agar plates in the samplers, two kinds of major mite allergens (Der I and Der II) were immunochemically quantitated. We found that the aerodynamic diameters of both allergens were mainly above 5.5 microns, and that airborne allergen levels decreased to about 10% of the starting level in 30 minutes, indicating the rapidity of the falling of both allergens.