Sera from 72 patients--29 females, 43 males--with symptoms suggestive of farmer's lung were tested by the Ouchterlony double diffusion technique using 10 different antigen solutions (extracts from mycetes and thermophilic actinomycetes, pigeon serum, extracts from pigeon droppings and Sitophilus granarius). In 23 cases precipitating antibodies were observed and Micropolyspora faeni was the most common antigen responsible for positive reactions. In 17 seropositive patients the diagnosis of exogen allergic alveolitis was established by clinical and laboratory findings. Measurements of immunoglobulins in these sera showed pronounced elevation of IgG and slightly elevated levels of IgA; IgM, IgE and C 3 c levels were in the normal range. The results are discussed with regard to the classification of farmer's lung as an occupational disease in Austria since December 30th, 1980.