We already showed that injection of Shiga toxin (Stx) 2 into mice caused severe granulocytosis in the peripheral blood. In this study we further clarified changes of granulocyte function by Stx 2. The activity of medullasin, a neutral serine protease in granulocytes that injures endothelial cells in vessels, significantly increased when Stx 2 was injected into mice intraperitoneally. Since granulocyte count in the peripheral blood of mice was markedly increased after intraperitoneal injection of Stx 2, medullasin activity in the peripheral blood was remarkably elevated. In contrast to Stx 2, injection of Stx 1 into mice caused no elevation of medullasin activity in granulocytes nor increase in granulocyte count in the peripheral blood. Cathepsin G levels in granulocytes increased only slightly after Stx 2 injection. Granulocytes obtained from mice injected with Stx 2 showed reduced superoxide-producing activity compared with those from controls. Addition of Stx 2 or Stx 1 to human mature granulocytes in vitro decreased their superoxide-producing activity when stimulated with agonists. Therefore, these toxins produced from Escherichia coli augment toxic effect of the bacteria by reducing bactericidal activity of granulocytes. Tissue injury in organisms infected with Shiga toxin-producing E. coli is mainly derived from elevated neutral proteases, such as medullasin, in granulocytes rather than direct toxic effect of superoxide from granulocytes. Hemolytic uremic syndrome caused by Shiga toxin-producing E. coli infection is due, at least in part, to the elevation of medullasin levels produced by granulocytes.