Successful triple-therapy treatment for Helicobacter pylori infection depends upon metronidazole (Mz) susceptibility, and many hospital laboratories routinely screen H. pylori isolates for Mz resistance using disc diffusion methods. We report the importance of culture medium when testing for metronidazole susceptibility. In this laboratory, Mz resistance in strains of H. pylori from patients in our area was found in approximately 80%. In other areas, Mz resistance is found in approximately 30%. This high rate of Mz resistance was not reflected clinically. Added haemin (X factor) and menadione in the culture medium drastically reduced zone size to Mz and also interfered with the minimal inhibitory concentration (MIC) as determined by Etest. When strains of H. pylori were re-tested on media which did not contain X factor or menadione, Mz resistance fell from 80% to 39%, a level similar to that seen in other areas.