The authors examined 23 precipitin-positive symptomatic patients with Farmer's Lung(FL) and compared them to different groups of exposed asymptomatic precipitin-positive(EAPP) and precipitin- negative(EAPN) farmers. The sera were tested using several techniques (i.e., immunodiffusion and ELISA for specific antibodies; polyethylene glycol [PEG] for circulating immune complexes [CIC]) in an attempt to find an in vitro test correlated with the disease which could also provide an insight into the pathogenic mechanisms of Farmer's Lung. Circulating immune complexes formed by IgG were significantly higher in Farmer's Lung patients than in EAPP subjects. In polyethlyene glycol precipitates from Farmer's Lung patients, specific antibodies found by ELISA correlated well with serum positivity, but they were not found in EAPP subjects. The possibility that the circulating immune complexes found were Ig aggregates was ruled out, as was the possibility that the antibodies found in the polyethylene glycol precipitate were also due to an unspecific link. The authors suggest that the circulating immune complexes of Farmer's Lung patients contain specific specific antibodies and that since their composition is different in EAPP subjects, these circulating immune complexes may play a role in the pathogenesis of the disease.