Fundamental and clinical studies were made on cefadroxil, a new oral cephalosporin, and the following results were obtained. (1) Antibacterial activity of the drug against S. aureus, S. epidermidis, E. coli, Klebsiella, Salmonella and P. mirabilis was almost equal to that of cephalexin. The MIC of indole positive Proteus. Enterobacter, Citrobacter, S. marcescens and P. aeruginosa to cefadroxil was higher than 100 microgram/ml in almost all strains. (2) Serum concentrations following an oral administration of 10.0 to 14.3 mg/kg of cefadroxil dry syrup was highest at 2 hours in 2 cases and 1 hour in 1 case, respectively, which were 13.4 to 17.1 microgram/ml, and 1.8 to 6.8 microgram/ml at 4 hours with an T 1/2 of 1.04 to 1.62 hours and apparently longer continuation of serum concentration than that of cephalexin. Urinary recovery rate was 75-96% up to 6 hours. (3) Fourteen patients, i.e., 6 with tonsillitis and 8 with urinary tract infection, were treated with a daily oral dose of 30-50 mg/kg divided in 4 doses except 1 case divided in 3 doses. The overall efficacy rate was 100%, i.e., excellent in 13, good in 1 and no failure. Causative organisms disappeared in all cases. (4) Adverse reactions, such as diarrhea and skin rash, were not noted at all and 1 case presented a mild elevation of GOT and GPT. (5) Taste and flavor of the drug was well palatable to children. (6) Based on the above results, it is concluded cefadroxil dry syrup is a new potent cephalosporin for oral use in the treatment of acute bacterial infection in children. Daily dose of 40 mg/kg in 3-4 divided doses appeared to be appropriate.