The authors have carried out the laboratory and clinical studies of cefadroxil (CDX). The results were as follows; The sensitivity was measured by plate dilution method on 27 strains of S. aureus and E. coli, 26 strains of K. pneumoniae isolated from patients. The distribution of sensitivity of S. aureus was 3.13-12.5 microgram/ml and the peak of distribution was 3.13 microgram/ml and 6.25 microgram/ml, of E. coli was 6.25 microgram/ml by 10(8) cells/ml. And the distribution of sensitivity of K. pneumoniae was 6.25-25 microgram/ml and its peak was 6.25 microgram/ml by 10(8) cells/ml. CDX were given orally at dose of 10 mg/kg to 3 children. The serum levels of CDX were 10.5 +/- 1.78 microgram/ml, 15.8 +/- 3.25 microgram/ml, 12.0 +/- 0.41 microgram/ml and 3.9 +/- 0.9 microgram/ml at 0.5, 1, 2, 4 hours after administration respectively, and was 2.3 +/- 0.48 microgram/ml at 6 hours. The urinary excretion rate was 55.6% up to 8 hours after administration. CDX were administered to 39 cases of pediatric infectious disease (26 cases with tonsillitis, 5 cases with enterocolitis, 3 cases with UTI, 2 cases with impetigo, each one case with bronchitis, cervical lymphadenitis and epididymitis). And CDX were given 25.0-65.2 mg/kg daily. Clinical results obtained were above good in all cases. No side effects were observed in any cases, except for one case, with diarrhea, 6 cases with the elevation of serum transaminase and 1 case with eosinophilia.