We present a case of Farmer's lung with antibodies to Thermoactinomyces vulgaris and Aspergillus fumigatus. A 56-year-old male patient with an atopic family medical history came to our hospital complaining of cough, dyspnea, fever, asthenia and anorexia. His condition worsened after being exposed to cereal powder, becoming symptomatic after 20 minutes or on occasion after 6 hours. Physical examination showed basal crepitant rales in lung auscultation. The radiograph of the thorax showed a bilateral interstitial reticulo-nodular pattern. An obstructive pattern was found on functional respiratory examination. There was also a slight restriction along with the decrease of the VC and a significant reversal of the M.M.E.F. with anticholinergics. The tests for intradermal cutaneous allergies were positive after 20 minutes and for Aspergillus fumigatus Niger and Terreus (Bencard) after 6 hours. With immunoelectrophoresis and double diffusion, precipitation bands in the presence of Aspergillus fumigatus and Thermoactinomyces, were detected. IgG and IgE were high-1570 mg% and 1000 U/ml respectively. The histological study of the transbronchial biopsy showed dilatation of the alveolar septum caused by a lymphocytic infiltration with fragments of collagenous fibres. In bronchoalveolar lavage there was a predominance of lymphocytes and histiocytes. After exertion, arterial blood gases showed desaturation with hypoxemia. The static lung volumes and the flow and diffusion of carbon monoxide (CO) showed a moderate decrease of vital capacity, with the total lung capacity being below normal. Diffusion was markedly attenuated. The provocation test by indirect bronchial inhalation using cereal powder (Alfalfa) was positive. After six hours dyspnea, cough and leukocytosis appeared with an outbreak of fever and an increase in the erythrocyte sedimentation rate.(ABSTRACT TRUNCATED AT 250 WORDS)