Pharmacokinetic and clinical studies on cefotiam (CTM) in mature neonates were carried out. The results were summarized as follows: The serum peak level of CTM after intravenous bolus injection at a single dose of 10 mg/kg was found at 15 minutes after the injection. The serum peak level was 32.9 micrograms/ml in a 1 day-old neonate and it was 17.7 micrograms/ml in a 4 day-old neonate. Serum levels at 6 hours after injection were 4.5 micrograms/ml and 0.7 microgram/ml for the 1 day-old and the 4 day-old, respectively. Half-lives were 2.1 and 1.2 hours in the 1 and 4 day-old neonates, respectively. Serum peak levels of CTM at 15 minutes after intravenous bolus injection at a single dose of 20 mg/kg were 40.9 micrograms/ml in a 1 day-old neonate and 36.5 micrograms/ml in a 5 day-old neonate. Serum levels of CTM at 6 hours were 8.0 micrograms/ml in the 1 day-old neonate and 2.3 micrograms/ml in the 5 day-old neonate. Half-lives were 2.5 and 1.5 hours in the 1 and 5 day-old neonates, respectively. With each dosage, the younger showed extended half-lives. A dose-response relationship was observed. In 2 cases of 2 day-old neonates given CTM 20 mg/kg by 30-minute intravenous drip infusion, the mean peak concentration at the termination of the infusion was 25.1 micrograms/ml. Even after 6 hours the concentration was found at 8.7 micrograms/ml. Half-lives were 2.9 and 3.7 hours. Urinary excretion rates of CTM in 1 to 5 day-old neonates were as low as about 20% in any of cases subjected to a 10 mg/kg intravenous bolus injection, a 20 mg/kg intravenous bolus injection a 20 mg/kg 30-minute intravenous drip infusion. It was possible to evaluate the efficacy of CTM in only 1 case of pneumonia. CTM was clinically and bacteriologically effective in this case. No abnormal clinical symptoms and findings were observed in all of the 5 cases.