Cefoxitin and cefamandole were evaluated in vitro against 263 organisms. Studies were performed in Mueller-Hinton and nutrient broth and agar employing inoculum sizes of 10(6) and 10(8) organisms per ml. At obtainable serum levels both antibiotics were bactericidal for nearly all strains of Escherichia coli, Klebsiella, Proteus mirabilis, and Staphylococcus aureus but were inactive against Pseudomonas aeruginosa and enterococcus. In agar, cefamandole appeared to be active against most strains of Enterobacter and indole-positive Proteus, whereas cefoxitin was active against indole-positive Proteus but not Enterobacter. Moreover, in broth medium most strains of Enterobacter were not readily inhibited by either antibiotic and only 40 and 73% of indole-positive Proteus were inhibited by 10 mug of cefamandole per ml in Mueller-Hinton and nutrient broth, respectively. However, in both broth media, 10 mug of cefoxitin per ml continued to be inhibitory and bactericidal for most isolates of indole-positive Proteus. Cefoxitin also was bactericidal against four cephalothin-resistant strains of E. coli. These data suggest that cefoxitin broadens the spectrum of existing cephalosporins by enhancing the activity against indole-positive Proteus species as well as some other Enterobacteriaceae. On the other hand, with the exception of strains of Enterobacter aerogenes, the apparent increased in vitro activity of cefamandole was demonstrated in agar and not in broth.