Cefmenoxime is a third-generation cephalosporin antibiotic. Cefmenoxime exhibited high activity against a wide variety of gram-positive and gram-negative bacteria. The in vitro activity of cefmenoxime against Streptococcus pyogenes, Haemophilus influenzae, and Enterobacteriaceae, including indole-positive Proteus, Serratia marcescens, Enterobacter cloacae, and Citrobacter freundii, was 10 to 1,000 times greater than that of several other cephalosporins. Against Pseudomonas aeruginosa, cefmenoxime showed activity two to four times that of sulbenicillin and carbenicillin but less than that of cefsulodin. Cefmenoxime showed good therapeutic activity against infection models in mice such as respiratory tract infection caused by Klebsiella pneumoniae and urinary tract infection caused by Proteus mirabilis.