Clinical studies of ceftriaxone (CTRX) were performed at a dose of 40 mg/kg once daily to evaluate its pharmacokinetics, and clinical and bacteriological efficacies in pediatric patients with respiratory tract infections. The following results were obtained. 1. Of 45 patients, clinical responses to CTRX were excellent in 34 (75.6%), good in 9 (20.0%) and poor in 2 (4.4%), indicating the overall efficacy rate of 95.6%. 2. Haemophilus influenzae (23 strains), Streptococcus pneumoniae (20 strains) and Moraxella catarrhalis (17 strains) were isolated from the patients as the main causative organisms. MIC90 of CTRX against these detected bacteria was < or = 0.06 microgram/ml with H. influenzae [beta-lactamase (-)/ABPC (S)], 0.25 microgram/ml with H. influenzae (BLNAR), 0.05 microgram/ml with PSSP, 1.0 microgram/ml with PISP/PRSP and 2.0 micrograms/ml with M. catarrhalis, respectively. 3. The eradication rate of causative organisms was 90.0% (27/30). 4. Serum levels of CTRX after administration of a 1-hour intravenous drip infusion of 40 mg/kg were investigated in 12 patients. Mean serum level at 24 hours after the administration was 9.4 +/- 2.8 micrograms/ml, which covered the level of MIC90 throughout the 24 hours. 5. No adverse reactions related to CTRX were observed. As the approved dosage of CTRX in pediatric patients is twice daily, while it is once daily in adults, there have been few reports on the efficacy of once-daily CTRX in pediatrics. According to the results of the study, it is suggested that once-daily CTRX for the pediatric patients with respiratory tract infections is useful. Further studies might be required to establish outpatient parenteral antibiotic therapy (OPAT) in pediatric infections.