The major obstacle for the successful measurement of airborne mite allergen is its very low concentration in the absence of vigorous disturbance. The aim of this study was to investigate the particle size distribution of group 2 dust mite allergen using an amplified ELISA system. Air sampling was performed using an Andersen sampler placed in the centre of the room, 1.2 m above floor level (airflow rate 28.7 l/min). This is a multistage, multiorifice cascade impactor that is comprised of six stages. Any particle greater that 4.7 μm should impact on stages 1 and 2, whilst stages 3-6 measure the predominantly respiratory range. The sampling was carried out for 30 min after 15 min of vigorous disturbance (vacuum cleaning without bag and filter). Der p 2 was measured using mAb-based ELISA with the AmpliQ amplification kit (Dako Ltd, Cambridgeshire, UK). The sensitivity was increased 15-fold as compared with standard assay, bringing the level of detection to 300 pg/ml. The majority of airborne Der p 2 (79.4%) was carried on large particles (> 4.7 μm). However, a small but important proportion of airborne Der p 2 (20.6%) was associated with small particles (1.1-4.7 μm). It is worth noting that all the levels measured were below the detection limit of standard assay. In conclusion, we have shown that using an amplification system, airborne mite allergen previously undetectable owing to its low concentration can be quantified. Group 2 dust mite allergen is carried not only on large particles. A small, but potentially significant proportion of this airborne allergen is associated with small particles which, when inhaled, may penetrate deep into the human respiratory tract.