House-dust mites (Pyroglyphidae) are an important source of indoor airborne allergens. Several methods may be applied to reduce the population growth of these mites and thus the quantity of allergen formed. One such method is to interfere with the mites' food chain. Fungi are a key factor in this food chain: they serve as an indirect food source. In this study we investigated the results of the repeated application of a fungicide (natamycin) on mattresses. As controls we treated some mattresses with a placebo, while others were left untreated. The application of natamycin appeared to hamper mite development. Additional vacuuming reduced the quantity of mite allergens present. In the usual household situation repeated treatment will be necessary to obtain a long-term reduction.