As a result of conducting experimental and clinical tests with the newly developed cephalosporin, cefoperazone (CPZ), the following conclusions were obtained: (1) When tested against 10 strains of Staphylococcus aureus and 16 strains of Staphylococcus epidermidis, the antibacterial activity of CPZ was found to be weaker than that of CEZ. Against 5 strains of A-beta-Streptococcus and 4 strains of Streptococcus pneumoniae, both CPZ and CEZ exhibited similar excellent antibacterial activity. CPZ was effective against 18 strains of Escherichia coli though its activity was influenced by the amount of inoculated bacteria present. Against 15 strains of Haemophilus influenzae and 10 strains of Haemophilus parahaemolyticus, CPZ was found to be more effective than CEZ though several high-resistant strains were noted. CPZ also showed more excellent antibacterial activity than CEZ against 4 strains of Haemophilus parainfluenzae, 5 strains of Klebsiella pneumoniae, 8 strains of Salmonella sp., 4 strains of Pseudomonas aeruginosa and 4 strains of Proteus sp. (2) The mean half-life in the blood following intravenous injections of 25 mg/kg and 10 mg/kg of CPZ to three children was 70 minutes. (3) One hour after intravenous injection of 25 mg/kg of CPZ to 3 cases of aseptic meningitis, drug concentration in the cerebrospinal fluid (CSF) was 1.20 mcg/ml, less than 0.39 mcg/ml and 1.55 mcg/ml. In one case, the CSF/serum ratio was 2.7%. (4) The average recovery rate in the urine of children who had received intravenous administrations of 25 mg/kg (3 children) and 10 mg/kg (1 child) was 17.8% between 0 and 6 hours. (5) Eighteen pediatric patients received CPZ in doses ranging from 48 to 170 mg/kg divided three-four times a day. They were RTI in 7, URI in 5, UTI in 5, SSSS in 1 and enteritis in 1 children. The clinical effectiveness of CPZ was judged to be remarkedly effective in 11 children, effective in 5 children and ineffective in 3 children, with an overall effective rate of 84.2%. One patient of tonsillitis combined sinusitis was considered 2 cases. The three cases in which the drug was found to e ineffective were 2 cases of pyothorax and 1 case of sinusitis. (6) Side effects were 1 case of eosinophilia, 2 cases of elevation of GOT and GPT, and 1 case of mild elevation of GOT. All were considered to be minor.