A 44-year-old farmer had respiratory symptoms and bibasilar pulmonary infiltrates after three exposures to a new biologic forage inoculant. Open lung biopsy revealed chronic interstitial pneumonitis and bronchiolitis obliterans with organizing pneumonia. The patient responded to oral corticosteroids but acutely worsened after an inadvertent reexposure to the forage inoculant. He later recovered, with return of lung function and chest radiograph toward normal. This case suggests that biologic forage inoculants may be associated with hypersensitivity pneumonitis.