Pharmacokinetic and clinical studies on ceftriaxone (CTRX) in mature and premature neonates were carried out. The results are summarized as follows. The mean serum peak level of CTRX after intravenous administration at a single dose of 10 mg/kg in 4 to 13 day-old-neonates was 34.1 +/- 11.4 micrograms/ml at 30 minutes. The mean serum level at 12 hours after dosage was 11.1 +/- 2.1 micrograms/ml. The mean half-life time was 11.6 +/- 1.4 hours. Mean serum peak levels of CTRX after intravenous administration at a single dose of 20 mg/kg were 64.6 +/- 15.4 micrograms/ml in 1 to 3 day-old-neonates, 44.6 +/- 1.1 micrograms/ml in 5 day-old-neonates at 30 minutes. Mean serum levels at 12 hours after dosage in these two groups of neonates were 16.4 +/- 4.8 micrograms/ml and 12.4 +/- 2.5 micrograms/ml, respectively. Mean half-life times were 13.7 +/- 3.7 hours in 1 to 3 day-old-neonates and 10.2 +/- 1.3 hours in 5 day-old-neonates. CTRX was clinically effective in a case of urinary tract infection. No side effect was observed except an elevation of GOT.