MICs of CB-183,315, a novel lipopeptide antibiotic, vancomycin, and metronidazole were determined for intestinal anaerobes and Enterobacteriaceae. The MIC(90)s for Gram-negative anaerobes were >8,192, 8,192, and 4 μg/ml for CB-183,315, vancomycin, and metronidazole, respectively. Against Enterobacteriaceae, the MIC(90)s were >8,192 μg/ml, 1,024 μg/ml, and 1,024 μg/ml, respectively. The CB-183,315 MIC(90) for Clostridium difficile was 0.5 μg/ml. Its lack of activity against normal fecal organisms makes it a promising new agent for treating C. difficile.