Reactive oxygen species (ROS) generated by Helicobacter pylori infection have been suggested to be important factors in induction of gastric malignancies. Utilizing electron spin resonance spectrometry, H. pylori-dependent radical formation and hydroxyl- and superoxide-anion radical scavenging activity was investigated. In contrast to previous reports, we found that H. pylori does not produce ROS, but displays superoxide scavenging activity. This scavenging activity was increased in cag-positive H. pylori strains when compared to strains lacking an intact cag pathogenicity island, and was dependent on enzyme activity. We hypothesize that the increased scavenging activity of cag-positive H. pylori strains is an adaptation to the increased inflammatory response associated with the cag-positive genotype of H. pylori.