Six subjects with farmer's lung underwent double inhalation challenge tests, each lasting 60 min, using natural antigen exposure. Subjects underwent the tests first without and then with protection by a particle-filtering half mask. Our purpose was to determine whether and to what extent the use of such masks reduced or prevented symptoms in affected patients. Clinical assessment included general and pulmonary symptoms, HR, temperature, WBC count, R, ITGV, VC, TLC, PO2, DCO, and chest x-ray film. The unprotected challenge provoked late responses characteristic of extrinsic allergic alveolitis. In the challenge using the mask, all six patients reported to be completely free of symptoms. Compared with the test without a mask, a statistically significant reduction in the rise of temperature, WBCs, HR, R, TLC, and PO2 was observed. Compared with the initial values, a statistically significant reduced increase of temperature and leukocytes was demonstrated in the test using the mask. Pulmonary and systemic alterations were significantly reduced but not completely prevented by the application of the particle-filtering half mask.