Abstract. Until now the detection of chymotrypsic or polysaccharidic arcs among precipitating systems indicated a positive diagnosis of Farmer's lung disease (FLD). It was shown that the occurrence of chymotrypsic activity is not absolutely correlated with the presence of clinical disease. The presence of different classes of specific precipitating antibodies was looked for and correlated with clinical symptoms in an attempt to find additional positive criteria for the diagnosis of FLD. The enzyme-linked immunofiltration assay (ELIFA) was adapted to detect the different specific antibody classes. 1400 serum samples were studied from 1350 farmers exposed to mouldy hay. Of 1358 samples examined from asymptomatic subjects tested as part of a systematic screening of the profession. 1356 had either no precipitins or have precipitins detected solely against non-chymotrypsic and non-polysaccharidic antigens. Ten specimens with precipitins joined the study. Only two serum samples from two symptomless individuals showed positive serological criteria for FLD as evidenced by the presence of a chymotrypsic band. The remaining forty-two serum samples from twenty-four patients with clinical signs of extrinsic allergic alveolitis had serological criteria for FLD as either chymotrypsic or polysaccharidic arcs. Only specific IgG in asymptomatic subjects was demonstrated, and when a supplementary class of specific antibody (IgM, IgA or IgE) was detected, there was clinical evidence of FLD in all except one case. Thus ELIFA could prove to be a useful tool for the positive diagnosis and prognosis of patients with FLD. In addition this method allows the recognition of a particular functional antigenic fraction of M. faeni by all classes of specific antibodies between different serum samples.