A 61 year old agricultural worker, without any respiratory past history, presented with a pulmonary fibrosis of rapid and fatal outcome. This was due to an extrinsic allergic fibrosing alveolitis of the Farmer's lung type. A search for precipitins "vis-à-vis" Micropolyspora faeni was strongly positive in the serum and broncho-alveolar lavage. Specific IgA antibodies to M. faeni, as well as specific antigenic fractions of M. faeni were demonstrated in the broncho-alveolar lavage. This IgA was almost certainly produced locally.