The antibacterial activity of a newly developed cephamycin antibiotic, MT-141, was compared to that of cefmetazole (CMZ) and latamoxef (LMOX), and the following results were obtained:<BR>1) MT-141 had a wide spectrum of antibacterial activity against gram-positive and gram-negative bacteria. The in vitro antibacterial activity (MIC) of MT-141 was slightly superior or similar to that of CMZ but was inferior to that of LMOX.<BR>2) MT-141 had a dose-dependent potent bactericidal activity against E. coli, K.pneumoniae, P. morganii and Serratia sp., and a markedly decreased cell counts at the early stage of its addition. Inhibition of regrowth after removal of MT-141 was slightly stronger than that of CMZ.<BR>3) When the relationship between MIC and MLC (minimal lethal concentration) was examined, it was found that 3-hour-MLC of MT-141 accorded with its MIC, and MT-141 exerted an excellent bactericidal activity within a short time.<BR>4) When the affinity for penicillin binding proteins of E. coli was examined, it was found that MT-141 had a marked affinity for PBP 1A, 1Bs and 3.<BR>5) The therapeutic effect was examined in mice infected with E. coli No.29, E. coli KC-14, K. pneumoniae KC-1, P. morganii 101, Serratia sp. GN 629, and S. marcescens T-55.The results revealed that MT-141 had a more excellent therapeutic effect than CMZ and LMOX.<BR>6) When fluctuations in number of bacteria were examined in the mouse peritoneal cavity, the data supporting the ED₅₀ of each drug were obtained.