Background and Aims: Helicobacter pylori surface proteins induce the production of proinflammatory mediators by mononuclear phagocytes, but the protein responsible for this stimulation has not been identified. This study determined whether urease, the major component of the soluble proteins extracted from H. pylori grown in culture, activates mononuclear phagocytes and stimulates them to produce proinflammatory cytokines. Methods: Primary human blood monocytes were incubated with column-purified H. pylori urease and assayed by flow cytometry, immunoassay, and reversetranscription polymerase chain reaction for phenotypic, functional, and molecular evidence of activation. Results: H. pylori urease induced monocyte expression of surface interleukin 2 receptors and increased expression of HLA-DR, phenotypic changes consistent with activation. Urease also stimulated dose- dependent production of interleukin 1β, interleukin 6, interleukin 8, and tumor necrosis factor α peptides and messenger RNA. These urease-induced phenotypic and functional changes were inhibited by preincubation of the urease with antisera to H. pylori whole bacteria, purified urease, or the 31- kilodalton subunit of urease. Conclusions: Among the soluble proteins released by H. pylori, urease is capable of activating monocytes for proinflammatory cytokine production. The local production of cytokines by urease-stimulated mononuclear phagocytes may play a central role in the development of H. pylori gastroduodenal inflammation.