Concentrations of birch-pollen antigens were measured in 10 homes in south-western Finland, four in urban and six in rural areas. Dust samples were collected once a week with a vacuum cleaner equipped with a special collection device (ALK, Copenhagen) combined with and exchangeable glass mirofiber filte in filte dish. Control samples were taken from horizontal surfaces outdoors. All samples were analysed by a modification of theIgG-Elisa procedure. the birch-pollen antigenic activity in indoor settled dust was lower than that in dust outdoors. The mean concentration of antigenic activity indoors peaked 3 weeks later than outdoors. The lag indicates that the most important means whereby antigens are carried indoors is via footwear and clothes, rather than, for instance, ventilation. Antigenic activity was still detected 2 months after the peak pollen period. As a source of antigens, both indoor and outdoor dust may be an important cause of pollen-allergy symptoms after the season.