In 4 of 17 Helicobacter pylori strains obtained from antral biopsy samples, the registered primary resistance (MIC, > 32 microgram/ml) appeared to be nonstable after prolonged microaerophilic incubation. In all resistant strains tested, susceptibility could be obtained when culture under normal microaerophilic conditions was preceded by a period of anaerobic incubation. Both of these findings may explain the observed discrepancy between the results of in vitro susceptibility tests and the eradication obtained in vivo.