Fundamental and clinical studies of ampicillin (ABPC) suppository (KS-R1) were performed in children with respiratory tract infections. Serum levels of ABPC after rectal administration of KS-R1 at a dose of 250 mg (11.4-17.7 mg/kg) in 3 children (3-5 years old) were 4.0-10.2 micrograms/ml (average 6.4 micrograms/ml) at 1/4 hour, 3.8-9.4 micrograms/ml (average 6.2 micrograms/ml) at 1/2 hour, 1.2-4.5 micrograms/ml (average 2.8 micrograms/ml) at 1 hour, 0.3-1.4 micrograms/ml (average 0.7 micrograms/ml) at 2 hours, and 0.3 microgram/ml in 1 case and unmeasurable amount in 2 cases at 4 hours. Urine concentrations of ABPC were 230-445 micrograms/ml (average 312 micrograms/ml) in 0-2 hours, 5.3-156 micrograms/ml (average 66.7 micrograms/ml) in 2-4 hours, and 1.3-13.4 micrograms/ml (average 6.1 micrograms/ml) in 4-6 hours, and the recovery rate into urine was 6.6-27.7% (average 15.6%) up to 6 hours. Clinical effects of KS-R1 on 16 childish patients with respiratory tract infections (acute purulent tonsillitis in 9 cases, acute bronchitis in 5 cases, acute bronchopneumonia in 1 case and acute purulent otitis media in 1 case) were excellent in 13 cases, good in 2 cases and poor in 1 case, and the effective rate was 93.8%. Bacteriologically, 5 strains of S. pyogenes, 4 strains of S. pneumoniae and 3 strains of H. influenzae were all eradicated with eradication rate of 100%. Side effect was weak diarrhea in 1 case, but this diarrhea immediately disappeared with discontinuation of treatment. There was no abnormality of clinical laboratory findings. It was concluded that KS-R1 is a useful drug for the treatment of respiratory tract infections in children.