The efficiency (dust collection and recirculation) of a conventional upright vacuum cleaner with a prototype vacuum bag (I) with pore size of 0.1 micron was compared with a standard vacuum cleaner bag (II), in the homes of 11 atopic asthmatic subjects with a known allergy to house dust mite. Four filter vacuum cleaners were assessed in pairs-the Vax 2000 with the Vorwerk VK121 ET340 (in 10 homes), and the Nilfisk GS90 'Allergy Vac' with the Bosch maxima 43 (in nine homes). All of the selected homes were vacuumed throughout (carpets and soft furnishings) for a 15 min period with each of the cleaners, and air was sampled simultaneously at 21 min-1 (Casella personal sampler). The weight of dust retrieved was recorded, and a sample of sieved dust (2 g) and the air filters were extracted to determine the concentrations of the major allergen, Der pI, by ELISA (ALK). No significant difference was observed in either total weight of dust, or airborne and dust concentrations of Der pI between using the conventional cleaner with vacuum bags I, or vacuum bags II. The Vax and the Vorwerk filter vacuum cleaners produced no measurable airborne Der pI concentrations during use in any of the 10 homes tested, whilst the Nilfisk produced airborne Der pI in two, and the Bosch in one of the nine homes tested. The Vorwerk retrieved significantly more dust from the floors than the Vax (P < 0.002). There was no significant difference in dust retrieval between the Nilfisk and the Bosch. All of the filter vacuum cleaners investigated produced lower concentrations of airborne Der pI, compared to the conventional cleaner with or without a special dust bag. However, the amount of dust and concentration/amount of Der pI that these cleaners actually retrieved, varied and the Vorwerk appeared most effective overall.