The in vitro activity of NVP-PDF386 (VRC4887), a novel new peptide deformylase inhibitor, and those of levofloxacin and clarithromycin were tested against 21 isolates of Chlamydia pneumoniae. The MIC at which 90% of the isolates were inhibited and the minimal bactericidal concentration at which 90% of the isolates were killed by NVP-PDF386 for all isolates of C. pneumoniae were 0.008 micro g/ml (range, 0.008 to 0.015 micro g/ml) compared to 0.25 and 0.06 micro g/ml for levofloxacin and clarithromycin, respectively.