To 39 neonates and premature infants 1 to 28 days old with various bacterial infections or suspected bacterial infections and were nearing cure-stage via ceftriaxone (CTRX) therapy, CTRX 10 mg/kg or 20 mg/kg was intravenously administered by bolus injection, and the changes in serum concentrations and urinary recovery rates of the drug were examined. Because the number of cases included was small, a comparison study was conducted by classifying them into three groups; less than 4 days old, 4 to 7 days, and 8 days or older, rather than dividing them into groups of neonates and premature infants. Clinical evaluation was conducted in 10 male and 6 female cases 1 to 46 days old, whose diseases comprised 1 case each of purulent meningitis, septicemia, pyothorax, phlegmonous cellulitis, and staphylococcal scalded skin syndrome, plus 5 with bronchopneumonia and 6 with urinary tract infection. 1. Changes in serum concentrations and urinary recovery rates (1) Intravenous bolus injection of 10 mg/kg: Serum concentrations in all three groups were the highest immediately after the drug administration, ranging from 32.3 to 35.9 micrograms/ml, with no significant differences noted among the groups. The levels gradually declined thereafter in all groups; to 12.7 to 18.3 micrograms/ml at 6 hours and 8.4 to 13.2 micrograms/ml at 12 hours. Averaged blood half-lives of CTRX were 11.3, 8.8, and 17.3 hours. The urinary recovery rates in the first 6 hours in the 3 groups were 31.0, 27.9, and 26.0%, respectively. (2) Intravenous bolus injection with 20 mg/kg: Serum concentrations were the highest immediately after the drug administration in all 3 groups, ranging from 56.5 to 73.1 micrograms/ml. The levels gradually declined thereafter in all groups, but remained rather high at 17.9 to 21.1 micrograms/ml at 6 hours and 13.2 to 16.8 micrograms/ml at 12 hours. The older the patients were, the shorter the serum half-lives of CTRX were: 25.5 hours in the less than 4 day old group, 11.7 hours in the 4 to 7 day old group, and 10.5 hours in the 8 days or older group. The urinary recovery rates in the first 6 hours in the 3 groups were 25.5, 22.3, and 21.8%, respectively. 2. Clinical results Clinical evaluation was made in 16 cases. CTRX at 11.9 to 60.0 mg/kg/day, was administered once daily or in 2 divided doses, daily. In all cases, including those with presumed severe infections that included 1 case each of purulent meningitis, septicemia and pyothorax, the efficacy was good or excellent.(ABSTRACT TRUNCATED AT 400 WORDS)