The beds, carpets and furnishings in 15 houses were sprayed with a solution containing tannic acid and an acaricide in an attempt to reduce allergen concentrations. Dust was collected from these sites for 4 weeks following spraying and the mite allergen Der p I concentration was measured and compared with baseline concentrations. In a subgroup of houses, counts of live mites and estimates of aeroallergen were also made. Four untreated houses were monitored over the same period. In dust samples collected 3 days after spraying, the mean concentrations of Der p I in beds, carpets and furniture were 23%, 28% and 26% of the pretreatment levels. All these reductions were significant compared to untreated controls. Samples collected 4 weeks after treatment were not significantly different to baseline for each group. After the initial reduction, the rate of increase in allergen concentration was significantly greater in the spray-only beds than in the beds which had been both sprayed and fitted with occlusive covers. Both aeroallergen and live mites continued to be detected in houses after treatment with the spray. These studies suggest that such sprays are only temporarily effective when applied at the manufacturer's recommended volumes and additional approaches are required to control the bulk of allergens in houses.