Assays of mite allergens (Der p I, Der f I and Der II) in the air of houses became feasible with the use of a low-noise air sampler and a sensitive radioimmunoassay described previously. The levels of the airborne allergens Der I (Der p I + Der f I) and Der II in the living room of 10 houses during usual domestic life were very low, 29.5 and 6.3 pg/m3, respectively, with a Der I: Der II ratio of 4.7:1. At the time of bedmaking, they greatly increased, about 1,000-fold, to 30,900 and 12,600 pg/m3, respectively, with a Der I: Der II ratio of 2.5:1. The amounts of Der I and Der II in the floor dust of the living room were 2,040 and 2,690 ng/g of fine dust, respectively, with a Der I: Der II ratio of 0.8:1. Der I seemed more prone to become airborne than Der II.