Pharmacokinetic and clinical studies of flomoxef (FMOX) in neonates and premature infants were conducted, and the results obtained are summarized below. 1. Plasma concentrations of FMOX at 15 minutes after one shot intravenous injection of 20 mg/kg to 6 cases were in a rang of 33.0-69.9 micrograms/ml and half-lives (T 1/2's) were between 0.68 and 4.89 hours. The plasma concentration of FMOX at 15 minutes after one shot intravenous injection of 40 mg/kg to 1 case was 79.9 micrograms/ml and the half-life (T 1/2) was 2.45 hours. Drug concentrations in plasma upon 1-hour intravenous drip infusion were 71.1-114.0 micrograms/ml and T 1/2's were 1.64-3.41 hours. T 1/2 tended to be couse shorter as ages of babies increased. 2. Urinary excretion rates in the first 6 hours after one shot intravenous injection of FMOX 20 mg/kg to 1 case and 1-hour intravenous drip infusion of FMOX 40 mg/kg to 2 cases were 60.4%, and 27.2 and 55.3%, respectively. 3. Clinical effects of FMOX against 12 cases of bacterial infections were excellent in 6 cases, good in 5 cases and poor in 1 case, thus the clinical efficacy rate was 91.7%. FMOX was also given to 6 cases for prophylaxis and prophylactic effects were observed in all the cases. 4. No adverse effects were observed in the 21 cases examined, but elevations of S-GOT and S-GPT were found in 1 case. The abnormal laboratory test results were probably due to this drug.