BRL 25000, granules preparation containing 2 parts of amoxicillin (AMPC) and 1 part of clavulanic acid (CVA, beta-lactamase inhibitor) as its potassium salt, has been investigated fundamentally and clinically. An in vitro study of the antibacterial activity of BRL 25000 against clinically isolated S. aureus (34 strains) showed higher activity than for AMPC alone and demonstrated that CVA potentiated the activity of AMPC, showing a synergistic effect against beta-lactamase producing organisms. A total of 27 pediatric patients aged between 6 months and 13 years 8 months (23 with respiratory infections and 4 with urinary tract infections) were treated with a daily dose ranging from 31.7 to 54.5 mg/kg, divided into 3 or 4 doses a day for periods of 4-18 days. The clinical effect was evaluated as excellent in 26 cases, poor in 1 case and the efficacy ratio was therefore 96.3% (26/27). The bacteriological effect against 12 organisms isolated from 9 patients was studied and all were eradicated (12/12). A drug-related side effect was observed in only 1 patient who developed diarrhea on the 4th day of treatment which continued during the treatment for 10 days. However, no severe side effect and no abnormality related to the drug in laboratory findings were observed. From these results it is concluded that BRL 25000 will be a clinically effective drug in the treatment of mild and moderate infections in the pediatric field.