Basic and clinical studies were made on Cefmenoxime (CMX), a new cephalosporin antibiotic, and the following results were obtained. The serum concentration of CMX was examined in four healthy adults after administration of 250 mg of CMX by intramuscular injection, intravenous injection and one-hour intravenous drip infusion (cross over). In the case of intramuscular injection, the peak value of 5.9 micrograms/ml was obtained 30 minutes after administration, and the half-life in serum was 1.41 hours. In the case of intravenous drip infusion, injection, a concentration value of 11.1 micrograms/ml on the average was obtained after 15 minutes of administration, and the half-life in serum was 1.26 hours. In the case of intravenous drip infusion, the concentration was 12.4 micrograms/ml upon completion of drip infusion, and CMX disappeared from serum at a half-life of 0.94 hour. The urinary recovery up to 6 hours was from 60 to 70% in each The efficacy rate of this preparation was 100% for 4 cases of acute simple cystitis. The efficacy rate of CMX was 70% for 10 cases of complicated urinary tract infection; the 3 cases in which CMX was not effective were patients with a residual catheter and Pseudomonas persisting or appearing as superinfection. It was noted that Serratia, which was resistant to the conventional cephalosporin antibiotics, became negative. No subjective side effects due to the administration of this preparation were observed. As for abnormal laboratory findings, a slight and transient rise in transaminases was observed in one case. On the basis of the above-mentioned results, it was concluded that CMX is an effective preparation for the treatment of urinary tract infections.