Ceftazidime (CAZ) was evaluated in the treatment of infections in neonates and premature infants. In 1 mature neonate (age: 14 days) administered with 30.3 mg/kg of CAZ by a single bolus intravenous injection, the serum concentration of CAZ was 50 micrograms/ml at 30 minutes after the dosing, and the elimination half-life was 1.38 hours. The urinary recovery rate of administered CAZ during the first 6 hours was 52.9%. In 6 neonates (mean gestational period: 37.8 weeks, mean birth weight: 2,508 g, mean age: 3.7 days) administered with ca. 20 mg/kg of CAZ by single bolus intravenous injections, the mean serum concentration of CAZ was 63.6 micrograms/ml at 30 minutes after the dosing, and the half-life was 3.78 hours. In 3 out of the above 6 neonates, the mean urinary recovery rate of CAZ during the first 6 hours was 59.8%. Five neonates and 2 infants with infections were given 18.4-65.9 mg/kg of CAZ b.i.d.-q.i.d. by intravenous injections. One of these cases was excluded from an evaluation for clinical efficacy, because CAZ was given only for 2 days. In the remaining 6 cases, clinical efficacy was excellent in 1 case of urinary tract infection (E. coli + E. faecalis), good in 3 cases of pneumonia (P. aeruginosa + K. pneumoniae: 1, P. aeruginosa: 1, and causative organism unknown: 1), and poor in 1 case of sepsis (S. aureus). In 1 case of asphyxia neonatorum contaminated with meconium, CAZ was used prophylactically, resulting with no infection. No clinically adverse effect was observed. Also no abnormality was observed in laboratory analyses in the 6 cases.(ABSTRACT TRUNCATED AT 250 WORDS)