Evaluations of ceftazidime (CAZ) in a few different categories were carried out in neonates. Single doses of 20 mg/kg of CAZ were administered to 8 neonates (day-age range: 1-26) and 3 infants (day-age range: 45-119) by bolus intravenous injection. Mean serum concentrations of CAZ at 15, 30 min., 1, 2, 4 hours and 6 hours were 51.6 +/- 9.2, 48.1 +/- 8.7, 47.9 +/- 7.8, 38.2 +/- 6.5, 20.2 +/- 4.0 micrograms/ml, and 15.3 +/- 5.8 micrograms/ml, respectively, in the neonates, and 51.1 +/- 10.3, 44.7 +/- 6.8, 35.5 +/- 4.1, 21.4 +/- 2.0, 8.6 +/- 1.0 micrograms/ml and 3.5 +/- 0.8 micrograms/ml, respectively, in the infants. Mean half-lives of CAZ in serum were 2.87 +/- 0.77 hours in the neonates and 1.39 +/- 0.10 hours in the infants, and mean urinary recovery rates in the first 6 hours were 60.5 +/- 16.0%, and 76.8 +/- 39.6% in the neonates and the infants, respectively. When individual differences are taken into consideration, no significant difference exists among 30-minute serum concentrations of neonates of different day-ages, and these concentrations were not significantly different from those in infants and older children. Half-lives of CAZ in sera decreased rapidly with the advances of the day-ages of the neonates, and the half-life at an age of 1-month should be similar to that in older children. The CAZ was administered to 2 cases of suspected sepsis, 7 of acute pneumonia, 1 of acute pyelonephritis, 1 of cellulitis, and 2 of idiopathic respiratory distress syndrome, and clinical efficacies were excellent in all the cases except for 2 cases excluded from the assessment. S. pyogenes (1), E. coli (1) and S. aureus (1) suspected as causative organisms were eradicated by the treatment with CAZ. Neither clinical adverse effects nor abnormal laboratory findings were observed in any case. From the above results, CAZ is considered to be an antibiotic with high efficacy and safety in the treatment of neonates.