Forty-three newborn and young infants including 13 low-birth-weight (LBW) infants were treated with flomoxef (FMOX) and the clinical efficacy and side effect were evaluated. The ages of the patients ranged from 0 to 99 days, and their body weights from 797 to 9,000 g. Dose levels were 10.5 to 48.5 mg/kg every 6 to 8 hours for 3 to 12 days. Those patients who responded to the FMOX treatment included 8 infants with sepsis, 14 with suspected sepsis, 6 with intrauterine infection, 2 with meningitis, 7 with pneumonia, 1 with staphylococcal scalded skin syndrome, 1 with epididymitis and 4 with urinary tract infections. The results were excellent in 17 and good in 22 patients. The drug was well tolerated, although diarrhea occurred in 2, slightly elevated serum concentrations of transaminases in 2, and eosinophilia and thrombocytosis in 1 patient each. Pharmacokinetic studies on FMOX with 20 mg/kg dose were done in 19 patients including 8 LBW infants. Serum concentrations at 15 minutes after intravenous bolus injection in five 1- to 6-day-old LBW, five 1- to 6-day-old and four 8- to 19-day-old mature infants were 52.6, 52.7 and 58.0 micrograms/ml, respectively, and those at 4 hours were 22.1, 13.3 and 5.2 micrograms/ml, respectively. Serum half-lives of the drug were 3.93, 2.29 and 1.62 hours, respectively, and excretion rates of this drug into urine in the first 6 hours after administration were 30.4, 45.1 and 58.7%, respectively. Mean serum concentrations just after intravenous 1-hour drip infusion in three 8- to 54-day-old LBW and two 8- and 10-day-old mature infants, were 31.5 and 18.9 micrograms/ml, respectively, and those at 4 hours were 15.3 and 4.3 micrograms/ml, respectively. Serum half-lives of the drug were 2.88 and 1.75 hours, respectively, and excretion rates of the drug into urine in the first 6 hours were 22.6 and 47.5%, respectively. The cerebrospinal fluid level at 3 hours after a dose was 7.09 micrograms/ml on the second day of treatment in a patient with Staphylococcus aureus meningitis receiving 50 mg/kg of the drug every 6 hours per day. Its level at 1 hour after a dose was 3.52 micrograms/ml on the 8th day of treatment in the same patient. The influence of FMOX on the fecal flora was studied in 7 patients. The characteristic pattern observed during the drug administration was the disappearance of Bifidobacterium, the decrease or disappearance of Enterobacteriaceae and the preservation of Streptococcus.(ABSTRACT TRUNCATED AT 250 WORDS)