A 64-year-old woman with rheumatoid arthritis and treated with bucillamine presented with a productive cough. No obvious infiltration was detected in chest radiography, but CT revealed patchy ground glass opacities in bilateral lung fields. Her serum KL-6 level was elevated and transbronchial lung biopsy showed interstitial pneumonia. Drug lymphocyte stimulation test (DLST) for bucillamine was negative for blood lymphocytes, but positive for bronchoalveolar lavage (BAL) lymphocytes. The pneumonitis improved after the cessation of bucillamine. We therefore made a diagnosis of bucillamine-induced interstitial pneumonia. DLST with BAL lymphocytes is thus suggested to be useful for such diagnoses.<br>