Helicobacter pylori infection leads to different chronic diseases, suggesting that this bacterium can evade the host immune defense system. The ability to control lymphocyte proliferation may be a mechanism leading to the development of gastric pathologies. Our aim was to characterize the effects of mucosa-associated lymphoid tissue (MALT) associated H. pylori strains on lymphocyte proliferation. MATERIALS AND METHODS: We measured the in vitro proliferation of human lymphocytes originally from blood or tonsil samples in the presence or absence of viable bacteria or lysates. RESULTS: We showed that MALT lymphoma-associated strains are not likely to be directly responsible for anarchical B-cell proliferation in vitro. On the other hand, proliferation of prestimulated T lymphocytes was abolished in vitro by the presence of all H. pylori strains, whether associated with MALT lymphoma or not. CONCLUSION: Inhibition of T-cell proliferation may be of major importance in the gastric colonization and in the persistence of the infection. Furthermore, this inhibition may favor anarchical B-cell proliferation in vivo and predispose the host to gastric MALT lymphoma, whereas MALT-associated H. pylori strains do not appear to possess a specific capability to directly stimulate B-lymphocyte proliferation.