A 41-year-old male developed acute interstitial pneumonia after inhalation of Blasticidin S, an antibiotic effective against rice blast disease. He presented with diarrhea, followed by dry cough, dyspnea and fever. A chest roentgenogram showed bilateral diffuse ground glass appearance, superimposed by patchy shadows. Arterial blood gas analysis revealed severe hypoxemia. Bronchoalveolar lavage fluid specimen showed moderate increase in total cell count, lymphocytes, neutrophils and eosinophils with marked elevation of CD4/8 ratio. Antibiotic therapy with minocycline failed to improve his condition, and methylprednisolone pulse therapy followed by methylprednisolone (48 mg) resulted in clinical remission with resolution of the chest roentgenogram findings. Blasticidin S should be kept in mind as a causative agent of acute interstitial pneumonia.