Cefotaxime (CTX) was used in the treatment and prophylaxis of infections in neonates and immature infants. The following results were obtained. 1. Mean serum concentrations (bioassay) 30 minutes after a single intravenous injection of about 20 mg/kg of CTX were 44.5 mcg/ml in neonates and 47.2 mcg/ml in immature infants aged 0-3 days, 45.8 mcg/ml in neonates and 56.4 mcg/ml in an immature infant aged 4-7 days and 40.6 mcg/ml in neonates and 38.1 mcg/ml in immature infants aged 8 or more days. Six hour values were respectively 10.9 mcg/ml, 17.0 mcg/ml, 4.6 mcg/ml, 13.4 mcg/ml, 3.8 mcg/ml and 2.7 mcg/ml. 2. Mean serum concentration half-lives were 3.0 hours in neonates and 3.2 hours in immature infants aged 0-3 days, 1.8 hours in neonates and 3.2 hours in an immature infant aged 4-7 days, and 1.5 hours in neonates and 1.6 hours in immature infants aged 8 or more days. 3. Urinary recovery rates were 0.8-78.0% for 0-6 hours after treatment. 4. Adequate clinical efficacy can be expected by the intravenous injection of CTX in doses of 20 mg/kg 2 times daily, every 12 hours, in neonates and immature infants aged 0-3 days, 20 mg/kg 3 times daily, every 8 hours, in neonates and immature infants aged 4-7 days, and 20 mg/kg 3 to 4 times daily, every 6-8 hours, in neonates and immature infants aged 8 or more days. 5. The clinical efficacy of CTX was good in all 4 cases of sepsis (including suspected case), excellent in 1 case of urinary tract infection, and good in all 4 cases of fever of unknown origin for a cure rate of 100%. 6. Adverse reactions were not noted in any cases.