One hundred and seventy-one strains of anaerobes were tested for susceptibility to carbenicillin by using agar dilution, broth dilution, and two disk diffusion methods. The minimal inhibitory concentration (MIC) for 67% of 51 strains of Bacteroides fragilis , 7 of 9 strains of Bacteroides melaninogenicus , and all of 8 strains of Eubacterium was 100 μg or less per ml. The MICs of the remaining anaerobes were 50 μg or less per ml. The broth dilution results were felt to be the most accurate of the four methods utilized.