Cefmetazole (CMZ) exerted excellent antibacterial effect in vitro for E. coli, Klebsiella spp. and Staphylococcus spp. bacteria isolated from clinical specimens in high yield (more than 40%). The effect was better than that of the conventionally used cephalosporin antibiotic agents. CMZ was also effective for anaerobic bacteria, such as Bacteroides spp., and the antibacterial spectra were broader than those of CEZ and CET. 2. Clinical effects of CMZ were examined for the postoperative traumatic infections and postoperative peritonitis in the field of surgery, and it was found that the effect of CMZ was seen in more than 80% in the both A and B groups; that is, CMZ was administered to the A group as the first choice, and CMZ was administered to the subjects to which CEZ and CET and other penicillin antibiotics were administered in vain (B group). 3. No side effects and influence on the laboratory test values were observed, and it may be said that CMZ is a safe antibiotic agent in the field of surgery.