Studies were made on pharmacokinetics, clinical effects and influence on intestinal bacterial flora in neonates upon administration of flomoxef (FMOX), and the results obtained are summarized as follows: 1. Serum concentrations of FMOX after intravenous administration of 20 mg/kg/dose were 48.5 micrograms/ml in 30 minutes, 33.0 micrograms/ml in 1 hour and 7.3 micrograms/ml in 6 hours. The T 1/2 was 2.7 hours. FMOX at a dose of 40 mg/kg was given to only one new born baby on the 1st day after birth, and serum concentrations were 73.6 micrograms/ml in 30 minutes, 55.9 micrograms/ml in 1 hour and 16.9 micrograms/ml in 6 hours. The T 1/2 was 4.60 hours. 2. FMOX was administered to 21 neonates between 0 and 32 days of age. Clinical effects were evaluable in 8 of 21 cases. The results were excellent in 2 cases, good in 5 cases and poor in 1 case. Of the 21 cases, diarrhea in 1 case, elevation of eosinophile in 2 cases (9.5%), elevation of platelet count in 3 cases (14.3%), and elevation of GOT in 2 cases (9.5%) were recognized during treatment. 3. FMOX was detected in feces at levels of 0.84-44.4 micrograms/g. Except for a slight decrease in numbers of anaerobes, little fluctuations of intestinal flora were observed during treatment.