Twenty-two newborn and young infants, including 13 premature infants, were treated with ceftriaxone (CTRX) and the clinical efficacy and side effects were evaluated. Ages of the patients ranged from 0 to 106 days, and their body weights from 1.19 to 3.92 kg. Dose levels were 15 to 23 mg/kg every 12 to 24 hours for 2 to 13.5 days. Eighteen infants with sepsis and 1 infant with purulent coxitis were considered to have responded to the CTRX treatment. The results were excellent in 13 and good in 6 patients. The drug was well tolerated, although diarrhea occurred in 2 patients, eosinophilia in 6 patients, slightly elevated serum concentrations of transaminases in 2 patients and thrombocytosis in 1 among the 22 patients. The pharmacokinetic studies on CTRX were done in 8 patients including 3 premature infants. The ages ranged from 3 to 50 days, and body weight from 2.20 to 3.94 kg. Plasma concentrations 30 minutes after single 10 mg/kg intravenous bolus injection in two 4- to 5-day-old premature neonates were 48.4 and 50.0 micrograms/ml and those at 6 hours were 22.7 and 23.4 micrograms/ml, respectively. In 2 mature neonates, plasma levels were 42.2 and 39.1 micrograms/ml at 30 minutes and 23.4 and 26.6 micrograms/ml at 6 hours after single 20 mg/kg doses. In four 12- to 50-day-old patients, plasma concentrations ranged from 35.9 to 175.0 micrograms/ml at 30 minutes and from 21.9 to 32.8 micrograms/ml at 6 hours after multiple doses of 20 mg/kg intravenous bolus injection. The plasma half-lives of the drug ranged from 6.6 to 16.8 hours in these 8 patients. Excretion rates of this drug into urine within 12 hours were 21.4 to 63.4% in 7 patients. Urine concentrations of the drug in 34 samples collected at various times from the 7 patients ranged from 28.3 to 469.0 micrograms/ml. The cerebrospinal fluid level at 2 hours after a dose was 3.33 micrograms/ml on the 5th day of treatment in 1 patient with sepsis receiving 18 mg/kg of the drug every 12 hours. Its level at 3 hours after a dose was 6.07 micrograms/ml on the 6th day of treatment in another patient with aseptic meningitis receiving 20 mg/kg every 12 hours. The influence of CTRX on the fecal flora was studied in 3 patients receiving 20 mg/kg X 2/day. The characteristic pattern observed during the drug administration was the disappearance of Bifidobacterium and Enterobacteriaceae, the preservation of Streptococcus and Staphylococcus, and the increase in Candida.(ABSTRACT TRUNCATED AT 400 WORDS)