Pharmacokinetics and clinical efficacy of ceftazidime (CAZ) were evaluated in neonates. The results obtained are summarized below. After a bolus intravenous injection of 20 mg/kg CAZ to neonates, peak serum concentrations ranged 55.1-97.4 micrograms/ml with the mean half-life of 2.26 hours in neonates with birth weight of 2,500 g or more, and 59.3-60.7 micrograms/ml with half-lives ranging from 3.98 to 4.22 hours in neonates weighing less than 2,500 g at birth. In the time-course observation, it was noted that the half-life at 2 days after birth was longer than half-lives observed on 4 and 9 days after birth. Four cases were given CAZ for treatment, and 3 cases for prophylaxis of infections, and the clinical efficacy was either excellent or good in all of the cases. Neither adverse effects nor abnormal laboratory findings were observed except rash in 1 case. Intestinal flora was examined in 5 cases. It was found that the effect of CAZ was less than that of LMOX, although some delay was seen in the growth of intestinal flora. There was no vitamin K deficiency in any of 6 cases examined.