Pharmacokinetic and clinical studies on clarithromycin (TE-031, A-56268), a new macrolide antibiotic, were performed in the pediatric field. 1. Pharmacokinetic investigation We studied serum concentrations and urinary excretions after single oral administration of TE-031 granules and tablets. Doses were 1, 5, 10 and 15 mg/kg body weight in case of granules (10% TE-031) and 150 mg/kg body weight in case of tablets (50 mg TE-031, 3 tablets). As results, Tmax's were 1-2 hours after administration in case of both granules and tablets. Cmax's in cases of granules were 0.29 +/- 0.15 micrograms/ml in 1 mg/kg administration, 2.53 +/- 0.71 micrograms/ml in 5 mg/kg, 4.11 +/- 1.37 micrograms/ml in 10 mg/kg, 6.28 +/- 1.48 micrograms/ml in 15 mg/kg showing a dose dependency. T 1/2's were 1.8-6.5 hours in cases of 1, 5, 10, 15 mg/kg of granules and tablets. T 1/2's became longer with increased doses. Urinary recoveries were 9.4 +/- 2.4% to 31.6 +/- 19.0% in 6 hours after administration. 2. Clinical investigation Clinical study was carried out in 24 patients of respiratory infections. Clinical efficacies were excellent in 11 patients, good in 12 patients and fair in 1 patient. The clinical efficacy rate was 95.8%. No side effects were observed. The above results suggest that TE-031 is a useful oral antibiotic for treating pediatric respiratory infections, especially those due to Mycoplasma pneumoniae.