The association between house dust allergy and asthma has long been recognized, and it has been demonstrated that a major allergen in house dust is related to the presence of mites of the genus Dermatophagoides. Using extracts of mite culture for skin testing, as many as 10% of the population and up to 90% of allergic asthmatics give positive immediate reactions. Although mites may occasionally become airborne during bed-making, it has also been demonstrated that they 'secrete or excrete' some allergen. Recently, we have shown that up to three-quarters of the serum IgE antibodies to mites are directed against a major allergen-antigen P1 (molecular weight 24,000). Using a radioimmunoassay it is possible to measure the concentration of this glycoprotein in both dust samples and mite cultures. These measurements, which are reported here, show that more than 95% of the allergen accumulating in mite cultures is associated with faecal particles.