Latamoxef (LMOX) was used in the treatment and prophylaxis of infections in neonates and immature infants. The following results were obtained. Mean serum concentrations (bioassay) 30 minutes after a single intravenous injection of about 20 mg/kg of LMOX were 49.9 mcg/ml in neonates and 47.3 mcg/ml in immature infants aged 0--3 days, 54.1 mcg/ml in neonates and 60.6 mcg/ml in immature infants aged 4--7 days, 48.9 mcg/ml in neonates and 46.7 mcg/ml in immature infants aged 8--28 days and 62.1 mcg/ml in immature infants aged over 29 days. Six-hour values were 24.1 mcg/ml, 22.5 mcg/ml, 15.9 mcg/ml, 27.2 mcg/ml, 12.9 mcg/ml, 19.1 mcg/ml and 12.8 mcg/ml, respectively. Mean serum concentration half-lives were 6.70 hours in neonates and 8.16 hours in immature infants aged 0--3 days, 3.68 hours in neonates and 5.83 hours in immature infants aged 4--7 days, 3.06 hours in neonates and 4.47 hours in immature infants aged 8--28 days and 2.59 hours in immature infants aged over 29 days. Adequate clinical efficacy can be expected by the intravenous injection of LMOX in doses of 20 mg/kg 1--2 times daily, in neonates and immature infants aged 0--3 days, 20 mg/kg 2--3 times daily, in neonates and immature infants aged 4--7 days and 20 mg/kg 3 times daily, in neonates and immature infants aged 8--28 days. The clinical efficacy of LMOX was good in 5 cases of sepsis (including suspected cases), 5 cases of urinary tract infection, 2 cases of respiratory tract infection and 6 cases of intrauterine infection (including suspected cases). Only a case of respiratory tract infections due to P. aeruginosa was thought to be ineffective. Bleeding tendency was noted in 3 cases, which results from secondary vitamin K deficiency should be checked carefully during the administration of LMOX.