Studies on pharmacokinetics and clinical effects of flomoxef (FMOX), a parenteral oxacephem antibiotic, were carried out in neonates. The results obtained are summarized as follows. 1. Mean peak serum concentrations of FMOX upon single administrations at doses of 20 mg/kg and 40 mg/kg were 33.3 +/- 7.33 micrograms/ml and 68.9 micrograms/ml, respectively. 2. Mean urinary recovery rates of FMOX in the first 6 hours after administration of the above doses were 35.2% and 48.3%, respectively. 3. FMOX was administered to 4 cases including 1 prophylactic case, 1 case each with aspiration pneumonia and sepsis, hypodermic abscess of the head, and itrauterine infection, at a dose of 20-30 mg/kg 2 or 3 times a day. Clinically, excellent results were obtained in 3 cases including an methicillin-resistant Staphylococcus aureus case. 4. No side effects nor abnormal laboratory test results were observed.