BACKGROUND: Various methods of killing the house dust mite to reduce exposure to allergen are being promoted even though complete data on their effects on allergen concentrations are not available. A study was designed to measure the concentrations of the main house dust mite allergen Der pI in homes treated with either solidified benzyl benzoate (Acarosan) or liquid nitrogen. METHODS: Der pI concentrations were measured in dust collected from mattresses, bedroom carpets, and living room carpets in 10 houses treated with Acarosan and 10 houses treated with liquid nitrogen. Samples were collected before the treatment (in July 1990) and three and six months afterwards (October 1990 and January 1991). Forty untreated houses were concurrently sampled as controls. RESULTS: Der pI concentrations were similar in the three groups at baseline. No significant fall was seen in either of the two treated groups three or six months after treatment. Concentrations in the control houses increased significantly--twofold to threefold in dust sampled from mattresses and bedroom carpets between baseline and October 1990. This increase was not seen in either of the treated groups of houses, but there was no significant difference in the Derp pI concentrations in these houses and the control houses from any site at any time point. CONCLUSIONS: Neither Acarosan nor liquid nitrogen reduced the concentrations of Der pI for as long as six months after application. A small effect was probably present as the rise seen in control houses in the three month samples was not found in the treated houses. This effect, however, is likely to be of little clinical importance and also to be transient as the trend was lost by six months.