It is increasingly recognized that the inability of the immune system to clear H. pylori infection is caused by an inadequate immune response and is associated with chronic gastric inflammation. To further investigate the cellular immune response to H. pylori, we studied PBMC from 31 H. pylori antibody-negative and 16 H. pylori antibody-positive individuals for H. pylori-induced DNA synthesis, secretion of the Th1-type cytokine IFN-gamma and secretion of IL-12, a cytokine produced by bacteria-stimulated monocyte/macrophages and a potent inducer of antibacterial immune responses and Th1-type T cells. All experiments were performed using Y. enterocolitica 03 as control. Our results demonstrate that DNA synthesis, IFN-gamma production and IL-12 production induced by H. pylori or Y. enterocolitica 03 did not differ significantly between H. pylori antibody-positive and H. pylori antibody-negative individuals. However, in the H. pylori-positive group there was a tendency, although not statistically significant, to produce less IFN-gamma in response to H. pylori but not Y. enterocolitica. These results demonstrate that monocyte/macrophages from H. pylori-positive individuals secrete normal amounts of IL-12 upon bacterial challenge and suggest that the decreased production of IFN-gamma in H. pylori-positive individuals observed in previous studies is selective for H. pylori and not caused by decreased IL-12 secretion.