A cross-sectional epidemiological study was carried out on 2932 farmers (response rate 92%) living in Northern Italy. A questionnaire on respiratory symptoms was supplied, chest X-ray taken and simple spirometry tests made. Thirty-nine subjects reported attacks of breathlessness associated with fever after exposure to mouldy hay; thirty-six had a clinical history of Farmer's lung and X-ray and/or respiratory function changes indicative of chronic Farmer's lung; 113 were classified as having bronchial asthma. These 118 subjects and a random sample of 131 non-symptomatic subjects were examined by intracutaneous skin tests and double diffusion precipitation tests with antigens associated with Farmer's lung. All sera positive by precipitation were examined by immunoelectrophoresis, counterimmunoelectrophoresis, and C1q binding test, and Bovine conglutinin test in solid phase for the detection of circulating immunocomplexes. 27.8% cases of chronic Farmer's lung, 4.4% of cases of bronchial asthma and 2.3% of non-symptomatic subjects showed precipitating antibodies against Farmer's lung antigens. Of the precipitin-positive subjects, the majority had precipitating antibodies against Micropolyspora faeni. No differences in immunoelectrophoregrams in number and pattern of precipitation arcs for M. faeni were found between chronic Farmer's lung and bronchial asthma subjects. Circulating immune complexes were present in 50% of chronic Farmer's lung patients, 40% of those with bronchial asthma and 33.3% of non-symptomatic subjects. Three Farmer's lung subjects (9.3%) showed immediate skin-test reactions to rural environmental allergens.