The contribution of Helicobacter pylori urease, the vacuolating cytotoxin VacA, and the 128-kDa protein CagA to the stimulation of human granulocytes in terms of phagocytosis and oxidative burst was evaluated. Blood was incubated with H. pylori strains and corresponding isogenic mutants lacking either the large urease subunit (UreB) or an accessory urease protein (UreG) or VacA or CagA. Phagocytosis and oxidative burst were monitored by flow cytometry. The UreB-lacking mutant was phagocytosed more efficiently (P < .001) and induced significantly less oxidative burst (P < .001) than its parental strain or the UreG-lacking mutant, which produces an enzymatically inactive urease. Values of the other mutants did not differ greatly from those of their parental strain. These data indicate inflammatory effects of H. pylori urease causing inhibition of phagocytosis and stimulation of oxidative burst by a pathway being largely independent of ammonia production.