The antigenity of outdoor dust which has settled on surfaces has been a neglected subject in aerobiological and allergological studies, although this dust is undoubtedly one of the main sources of airborne antigenic particles which cause allergic symptoms. Dust samples for birch pollen antigen analyses were taken with a vacuum sampler from horizontal surfaces, and from birch and maple leaves once a week after the peak birch pollination period. The samples were analysed by a modification of the IgG-ELISA-procedure. The antigenicity of dust on all surfaces remained high for months after birch pollination, even on days when no pollen grains were microscopically detected in the dust samples. Outdoor dust being one of the sources of indoor dust is thus a component of the mixture which brings about an allergy to house dust. Dust with a high antigenicity, carried inside from outdoors may be one cause of the occurrence of allergic symptoms outside of the pollen season.