1. MIC of 6059-S against 92 strains of clinically isolated bacteria were measured. The compound was active against most of Gram-negative rods, but was not active against Staphylococcus aureus. 2. 20 mg/kg of 6059-S (newly synthesized oxacephem antibiotics) was administered to the pediatric patients and its blood concentration was measured by agar well method using E. coli 7437 as a test organism. 3. The mean blood concentrations were maximum at 15 minutes after intravenous one-bolus injection. Maximum levels were 94.5 mcg/ml in the patients of below 5 years old and 98.7 mcg/ml above 6 years old. Their half-life of the blood levels were 95.4 and 110.6 minutes respectively. 4. The mean blood concentrations were highest at the end of the infusion in the cases of 60 minutes drip injection. Maximum levels were 85.0 mcg/ml in the patients of below 5 years old and 64.8 mcg/ml above 6 years old. 5. Clinical efficacy of 6059-S in 6 cases pyelonephritis, 2 cases of sepsis, 1 case of meningitis, 1 case of intraperitoneal abscess, 9 cases pneumonia and 2 case of tonsillitis was 100%. In the case of urinary tract infection, 4 patients were treated successfully by the administration of 20 mg/kg/day of 6059-S. Other bacterial infections were treated with 55 to 200 mg/kg/day. 6. 100% of the causative organisms were eliminated by 6059-S. They were E. coli, Klebsiella pneumoniae, Serratia marcescens, H. influenzae and beta-Streptococcus. 7. No remarkable side effect was noticed during administration.