CASE PRESENTATION: Seventy-two year old male with idiopathic pulmonary fibrosis underwent bilateral lung transplant with an uneventful intraoperative course. Post-operative prophylactic antibiotics included vancomycin and piperacillin-tazobactam for 3 days, trimethoprim-sulfamethoxazole, valganciclovir, itraconazole and inhaled amphotericin B. Immunosuppressive regimen included tacrolimus, prednisone and mycophenolate mofetil. He was extubated on post-operative day 1. On day 10, he developed dyspnea, purulent sputum and leukocytosis. Empiric antibiotics were restarted. Chest computed tomography revealed diffuse septal thickening and ground-glass opacities concerning for rejection. Sputum stain showed polymorphonuclear cells but no organisms. Empiric methylprednisolone (7.5mg/kg) was added for presumed acute cellular rejection followed by intravenous (IV) immunoglobulin, plasmapheresis and rituximab for presumed antibody mediated rejection. Bronchoalveolar lavage (BAL) and transbronchial biopsy revealed A0B0 rejection with diffuse alveolar damage (DAD), but no infection. On day 27, intubation was required for progressive respiratory failure. Open lung biopsy revealed acute lung injury (ALI) with organizing pneumonia and DAD. Stains for fungal, viral and bacterial (including acid fast bacilli) organisms were initially negative. Ten days after inoculation, tissue culture revealed M hominis on anaerobic media. He was treated with IV doxycycline for 3 weeks. Subsequent BAL culture was negative for M hominis and he was discharged on 2L supplemental oxygen.