One hundred fourteen Clostridium difficile strains were collected from 67 patients and analyzed for the presence of C. difficile toxin B by the cell cytotoxoicity neutralization assay, genes for toxin A, toxin B, binary toxin and TcdC deletion by PCR. All strains were also PCR-ribotyped. The MICs of the isolates were determined against MCB3681 and nine other antimicrobial agents by the agar dilution method. All isolates were positive for toxin B as well as for toxin A and B genes. In addition, 13 isolates were positive for the binary toxin genes. Thirty-two different ribotypes were identified. No strain of ribotype 027 was found. All 114 isolates were sensitive to MCB3681 (0.008-0.5 mg/l), cadazolid (0.064-0.5 mg/l), fidaxomicin (0.008-0.125 mg/l), metronidazole (0.125-2 mg/l), vancomycin (0.125-1 mg/l) and tigecycline (0.032-0.25 mg/l). Three isolates were resistant to linezolid (8 mg/l), 12 isolates were resistant to moxifloxacin (8-32 mg/l), 87 isolates were resistant to clindamycin (8-256 mg/l) and 107 isolates were resistant to ciprofloxacin (8-256 mg/l). No association between toxins A, B and binary toxin, ribotypes and the sensitivity to MCB3681 could be found. MCB3681 has a potent in vitro activity against C. difficile.