Pharmacokinetics and clinical studies on ceftriaxone (CTRX) in pediatric surgery were performed and the results obtained are summarized below. 1. In neonates, serum and urinary levels of CTRX were measured in 4 patients following injection (CTRX 20 mg/kg, intravenous bolus injection). Highest levels in serum were observed at 15 or 30 minutes, 59.4-212.5 micrograms/ml. Serum levels of CTRX then decreased very slowly, and serum half-lives (T 1/2) were 7.9-27.1 hours. Urinary recovery rates were 17.0-54.7% in 12 hours. 2. Bile levels of CTRX were also measured in 8 patients with congenital biliary atresia and a patient with congenital biliary dilatation (CBD). Highest levels of CTRX in bile, 10.2 and 13.2 micrograms/ml, were noted in 2 hours following injection to 2 patients. But in other patients, CTRX was undetectable in bile. Recovery rate in bile in 12 hours in a CBD patient was 0.19%. 3. The CTRX was administered to 7 patients (a case of infected lung cyst, 2 cases of peritonitis and 4 cases as prophylaxis to postoperative infection). Clinical results were good in all cases. No clinical and laboratory adverse reaction due to the administration of CTRX was observed. It is concluded that CTRX is a safe and effective antibiotic in pediatric surgery. However, care has to be practiced in determining dosage and interval of CTRX administration because of its pharmacokinetical characteristics.