Thirteen neonates and young infants, including 5 infants with very low birth weight, were treated with ceftizoxime (CZX) and its clinical efficacy and side effects were evaluated. The ages of the patients ranged from 0 to 96 days, and their body weights ranged from 580 to 5,050 g. Doses given were 20-54 mg/kg every 6 to 12 hours for 2.5 to 7.5 days. Two infants with sepsis, one with urinary tract infection, one with sepsis and urinary tract infection, and 1 with fetal infection were considered to have responded satisfactorily to the CZX treatment. The drug was well tolerated and side effects was not apparent. Pharmacokinetic studies were done on CZX in 8 patients including 4 infants with very low birth weight. Their ages ranged from 2 to 91 days, and body weights from 545 to 5,050 g. Serum concentrations at 2 hours after single 20 mg/kg intravenous bolus injections were 19.2 to 44.2 micrograms/ml and the levels were 2.11 to 26.3 micrograms/ml at 8 hours. Elimination half-lives of CZX ranged 1.90 to 9.57 hours in these patients. In 2 infants with very low birth weights with ages 7 and 91 days, half-lives were as long as 9.57 and 8.24 hours, respectively. Urinary recovery in 6 hours was 31.9-66.9% in 5 patients. Urine concentrations of the drug in 24 samples collected at various time from the 7 patients ranged from 130 to 3,219 micrograms/ml. Influence of CZX on the fecal flora was studied in 1 patient given 20 mg/kg X 4/day of the drug.(ABSTRACT TRUNCATED AT 250 WORDS)