Doses of 50 mg of ceftazidime per kg were administered intravenously to 29 newborn infants every 8 or 12 h for 3 to 5 days. Mean peak concentrations in plasma ranged from 102 to 124 micrograms/ml. Mean elimination half-life values ranged from 2.9 to 6.7 h and varied inversely with gestational age and plasma clearances. Peak and trough plasma bactericidal titers against an Escherichia coli and a group B streptococcus strain were at least 1:16 and 1:32, respectively.