Enzyme-linked immunosorbent assay (ELISA) was used to measure antibodies against Aspergillus fumigatus in farmers' lung patients, various other pulmonary diseases and healthy controls. Both the prevalence and the mean titre of antibodies were significantly higher in farmers' lung patients than in the other groups. There was no difference between controls and patients with bronchial asthma or non-allergic pulmonary diseases. On the other hand, in the group of patients with respiratory diseases of undefined aetiology the mean antibody titre was significantly higher than that of the controls. Comparison of ELISA and the precipitin test showed the higher sensitivity of ELISA, but otherwise the two tests were in a close agreement.