Clinical effects of SY5555 dry syrup, a new oral penem antibiotic, were analysed in 20 children with various bacterial infections. Ages of the patients varied from 8 months to 14 years. Doses of SY5555 were varied from 12.8 mg/kg/day to 30.5 mg/kg/day, and it was administered in 3 divided dosages. Clinical efficacy rates were as follows; 6/7 in acute bronchitis, 5/5 in pharyngotonsillitis, 3/3 in acute otitis media and 2/2 in cystitis and 3/3 in impetigo contagiosa. The overall rate was 95.0% (19/20). Bacteriologically, eradications were obtained with 1/2 strains of Streptococcus pyogenes, 3/3 of Staphylococcus aureus, 1/1 of Haemophilus influenzae, and each of Staphylococcus epidermidis, Haemophilus parainfluenzae, coagulase-negative staphylococci and Serratia marcescens. Diarrhea was observed in 1 patient. And elevated eosinophiles or GPT was observed in one patient each. In vivo pharmacokinetics of SY5555 was examined in 2 cases. Peak plasma levels were observed at 1 hour after dosage in one patient and at 2 hours in another upon oral administration of 8.3 mg/kg of SY5555, and peak levels were 2.44 and 1.38 micrograms/ml respectively. Half-lives of SY5555 were 1.39 and 0.59 hr. Concentrations of SY5555 in urine after administration were 70.2 (2-4 hrs.) to 91.0 (0-5 hrs.) micrograms/ml, respectively. SY5555 dry syrup is considered as an useful and safe antibiotic in treating the infectious diseases in children.