Flomoxef (FMOX, 6315-S) is a new oxacephem with a broad spectrum of antimicrobial activity. We used FMOX for treatment of 13 patients with respiratory tract infections including 4 cases of pneumonia, 5 of lung abscess and 4 of exacerbation of the chronic airway diseases. FMOX showed excellent in vitro antimicrobial activities against clinical isolates including 4 strains of Streptococcus pneumoniae, 2 strains of Haemophilus influenzae and each one strain of Escherichia coli and Klebsiella pneumoniae. Clinical responses were excellent in 3 cases, good in 7 and fair or poor in 3. No side effect was observed, but abnormal laboratory findings caused by FMOX administration were found in 2 cases; hypertransaminasemia and eosinophilia. However, neither of them was severe. From the above results, it is considered that FMOX will be useful for treatment of patients with respiratory tract infections.