Allergens in fine particles may cause symptoms in allergic asthmatics. In order to assess the exposure of susceptible persons, a method to measure the allergen load in fine and coarse particles was developed. Aerosols are collected with a high-volume air sampler by multistage impaction. They are separated into five size classes, ranging from > 10 μm to < 1 μm and sampled on glass fibre filters. After sampling, filters are crushed into a fine powder using a hydraulic press. Allergens are then eluted on a shaker into Tween-20-containing phosphate buffered saline. After microfiltration, the eluate is ready for analysis with ELISA-techniques (Enzyme Linked Immuno Sorbens Assay). Two different methods are used for the analysis of allergens: One is a sandwich-ELISA using monoclonal IgG-antibodies, the other is a competitive ELISA based on polyclonal IgE-antibodies obtained from patients allergic to birch pollen. Using the monoclonal antibodies information on the amount of one particular allergen (the major allergen Bet v1) is obtained. On the other hand the competitive ELISA using the polyclonal IgE is much more sensitive and indicates the total birch pollen allergens. Data obtained during spring 1998 show good correlation of pollen counts and allergen content in the coarse particle fraction containing intact pollen (> 10 μm). In smaller sized fractions, the allergen load is often close to the detection limit. When clearly detectable amounts of allergen are present, in the fine size fraction the allergen load shows only a weak correlation to the pollen counts and the allergen concentrations in the coarse particle fraction.