We tested the in vitro activity of levofloxacin, ciprofloxacin, doxycycline, tetracycline, erythromycin, roxithromycin, clarithromycin and azithromycin against 110 clinical isolates of Mycoplasma hominis. The minimal inhibitory concentrations (MICs) were determined with the Etest. The minimal concentrations at which 90% of the isolates were inhibited (MIC(90)) were 0.064 microg/ml doxycycline and 0.19 microg/ml tetracycline. In 9 isolates (8.1%), the MIC for doxycycline was 4-12 microg/ml. These isolates were also resistant to tetracycline with a MIC of 32-128 microg/ml. No significant difference was found between doxycycline and tetracycline (p = 0.076). Comparison of the two quinolones revealed that the MIC(90) for levofloxacin was 0.19 microg/ml and for ciprofloxacin, 0.5 microg/ml. A significant difference was found between doxycycline/tetracycline and levofloxacin or ciprofloxacin (p = 0.0001), and between levofloxacin and ciprofloxacin (p = 0.001). All the isolates were highly resistant to the macrolides with MIC > or = 256 microg/ml. This finding has important implications for cases in which Mycoplasma infection is suspected and culture and/or in vitro susceptibility tests are not available.