Antimicrobial activity of 3-carbamoyloxymethyl-7-alpha-methoxy-7-[2-(2-thienyl)-acetamido]-3-cephem-4-carboxylic acid (cefoxitin), a new semisynthetic cephamycin antibiotic, was studied in comparison with that of cefazolin and cefalotin. Cefoxitin exhibited antibacterial activity against both gramnegative and gram-positive bacteria, and its action was bactericidal. Against gram-negative bacteria, cefoxitin was highly active as well as cefazolin, and more active than cefalotin. Especially, cefoxitin was highly active not only against strains of clinical isolates of indole-positive Proteus and S. marcescens but also against those of E. coli and P. mirabilis which were resistant to cefazolin and/or cefalotin, respectively. In addition, cefoxitin was effective against the strains resistant to beta-lactam antibiotics including cefazolin and cefalotin. Cefoxitin was hardly active against the strains of E. cloacae and P. aeruginosa, similar to cefazolin and cefalotin. Against gram-postive bacteria, cefoxitin was less active than cefazolin and cefalotin. In protection tests in mice, cefoxitin and cefazolin were more effective against infection with E. coli than cefalotin. Furthermore, cefoxitin was more active against infections with S. marcescens and P. morgani than the other antibiotics. Cefoxitin, like cefalotin, was less effective against infection with S. aureus than cefazolin. Cefoxitin was highly resistant to hydrolysis by beta-lactamases derived from the organisms insusceptible to the antibiotic. This fact revealed that the resistance of the organisms to cefoxitin may be in part due to factors other than beta-lactamase inactivation.