Serum concentrations, urinary excretion and clinical responses of flomoxef (FMOX) were studied. The results are summarized as follows. 1. Serum concentrations of FMOX were 17.4 micrograms/ml 1-hour after intravenous injection on the average in 5 cases who received approximately 10 mg/kg, 41.8 micrograms/ml in 2 cases given 20 mg/kg, and 69.6 micrograms/ml in 2 cases given 40 mg/kg, indicating that serum concentrations of FMOX changed in a dose-dependent manner in this range. Average serum half-life (T 1/2) in 4 mature babies was 2.48 hours and that in 6 premature babies was 3.17 hours, indicating that elimination rates in premature cases tend to be slower than those in mature cases. Urinary recovery rates averaged 39.2% in the first 6 hours in 5 cases examined. 2. Five newborns or premature babies received FMOX 33.1-80.2 mg/kg (b.i.d. or t.i.d.) via intravenous route for 5 to 8 days. FMOX showed excellent or good clinical effectiveness in the treatment of all patients including 1 case each of sepsis with urinary infection, furunclal otitis, impetigo, uterogenic fetus infection and urinary infection. Bacteriological responses were also studied, and eradication of identified organisms (Escherichia coli 3 strains and Staphylococcus aureus 2 strains) was obtained upon the FMOX treatment, but in 1 strain of S. aureus showed only a decrease. No adverse reactions were observed in any cases, but a slight elevation of eosinophil was noted in 1 patient receiving a dose of 210 mg a day. From the results obtained in these tests, FMOX appears to be very usefull and safe for the treatment of some infectious diseases in neonates.