DN-9550 [(6R, 7R)-7-[(Z)-2-(2-aminothiazol-4-yl)-2-(1H-imidazol-4-yl) methoxyiminoacetamido]-3-[(1-pyridinio)methyl]-8-oxo-5-thia -1-azabicyclo methyl]-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate hydrochloride] is a new semisynthetic cephalosporin with a broad spectrum of antibacterial activity against gram-positive and gram-negative bacteria. The activity of DN-9550 against most species of the family Enterobacteriaceae was roughly comparable to that of ceftazidime, slightly lower than that of cefotaxime, and far exceeded that of cefoperazone. Against Citrobacter freundii, Enterobacter cloacae, and Serratia marcescens, DN-9550 was more active than ceftazidime and cefotaxime. DN-9550 and ceftazidime were significantly more active than cefotaxime against Pseudomonas aeruginosa, but DN-9550 and cefotaxime were clearly more active than ceftazidime against staphylococci and streptococci. Haemophilus influenzae and Neisseria gonorrhoeae were also highly susceptible to DN-9550, but Bacteroides fragilis was generally not susceptible to the compound. DN-9550 was stable to various types of beta-lactamases and had high affinities for penicillin-binding protein 3 of both Escherichia coli and P. aeruginosa. When DN-9550 was administered subcutaneously to mice experimentally infected with Staphylococcus aureus, Streptococcus pyogenes, Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, or Pseudomonas aeruginosa, its efficacy well reflected its in vitro potency.