The in vitro susceptibility of 27 clinical isolates of Helicobacter pylori to erythromycin, clarithromycin, azithromycin and temafloxacin under various pH conditions was evaluated. Clarithromycin (MIC90 0.03 micrograms/ml) was found to be significantly more active than either erythromycin (MIC90 0.125 micrograms/ml) or azithromycin (MIC90 0.25 micrograms/ml) at a neutral pH. Lowering the pH to 5.75 resulted in a loss in efficacy from 8- to 32-fold for all three macrolides studied. The MIC90 of clarithromycin (0.5 micrograms/ml) remained lower than those of azithromycin (2 micrograms/ml) and erythromycin (4 micrograms/ml). No synergism or antagonism was observed with combinations of clarithromycin and temafloxacin at either the neutral or lower pH values.