SY5555 is a new oral penem antibiotic. Pharmacokinetic and clinical studies using SY5555 dry syrup (powder which is dissolved before use) were performed in pediatric patients. 1. Pharmacokinetic investigation Peak plasma concentrations of SY5555 after dose of 5 mg/kg, 10 mg/kg and 15 mg/kg were, respectively, 1.58 +/- 0.37 micrograms/ml, 2.78 +/- 0.54 micrograms/ml and 5.28 micrograms/ml at 1 hour. The average half-life with 5 mg/kg administration was 0.94 +/- 0.05 hours, that with 10 mg/kg was 1.46 +/- 0.31 hours and that with 15 mg/kg was 0.88 hours. 2. Clinical investigation Enrolled in the study were 15 patients including 5 with acute otitis media, 5 with urinary tract infections and 1 each with pharyngitis, tonsillitis, bronchitis, pneumonia and subcutaneous abscess. Responses were excellent in 4 patients, good in 8 patients, fair in 2 patients and poor in 1 patient. In the assessment of the bacteriological efficacy, 8 out of 10 strains of organism identified previous to treatment were eradicated and 2 strains were unchanged, hence the eradication rate was 80.0%. 3. No adverse reactions attributable to the drug were observed and good drug compliance were obtained. From the above results, it has been concluded that SY5555 is a highly effective and safe agent for mild to moderate respiratory and urinary tract infections in children.