Seventy isolates of Helicobacter pylori from antral biopsy samples were tested for their susceptibilities to metronidazole by agar dilution. Seven (10%) of these clinical isolates appeared to be resistant to metronidazole. Sixty-three strains were susceptible. In 42 (67%) of the 63 susceptible isolates, resistant isolates were obtained by serial passage on plates containing subinhibitory concentrations of metronidazole. In 10 of these 42 strains, the acquired resistance appeared to be unstable. The difference between the stability of resistance that occurred after one or two passages and the stability of resistance that occurred after three passages was statistically significant (P < 0.006). Primary resistance in clinical isolates was a stable phenomenon. Whether the resistance that emerges during therapy in patients is stable or unstable needs to be established.