Helicobacter pylori is a Gram-negative bacterium that causes a variety of gastrointestinal diseases, such as duodenal ulcer and gastric carcinoma. The T cell response against H. pylori is thought to contribute to the pathogenesis of these diseases. Here, we show that mouse-adapted H. pylori is able to polyclonally activate murine CD4(+) T lymphocytes, irrespective of their antigen specificity. Murine T helper cell clones as well as short-term cultured, polyclonal Th1 and Th2 cell lines and a human T cell clone, but not naive CD4(+) T cells, could be activated in this manner. The effect was independent of antigen-presenting cells and required direct contact between H. pylori and T cells. Only whole cells of H. pylori, but not lysates or sonicates were able to activate T cells. The activity was lost after long-term culture of H. pylori on agar-plates. Degradation of H. pylori proteins with specific peptidases dramatically reduced the stimulating ability, implicating that the responsible molecule is likely to be a protein. This unexpected polyclonal T cell stimulatory mechanism may contribute to the T cell-mediated pathogenicity characteristic for H. pylori-mediated diseases.