The objective of these experiments was to examine the ability of Helicobacter pylori to stimulate interleukin-10 (IL-10) or IL-12 and select for either Th1 or Th2 cells. Gastric biopsy specimens were collected from patients who were categorized with respect to the presence of H. pylori and gastric disease as well as their age, gender, medications, and other factors. As Th1 and Th2 cells are selected by IL-12 and IL-10, respectively, biopsy specimens were screened for mRNA and protein for these cytokines. Although mRNA for IL-12 and IL-10 was detected in biopsy specimens obtained from both infected and uninfected patients, IL-12 protein predominated. Levels of IL-10 and IL-12 in gastric tissue did not change in response to infection. Moreover, gamma interferon (IFN-gamma)-producing T cells were found in both the infected and the uninfected gastric mucosa. Stimulation of peripheral blood leukocytes from either infected or uninfected donors with various concentrations of live or killed H. pylori induced immunoreactive IL-12 and IL-10. After stimulation with live H. pylori, IL-12 levels increased more than 30-fold, whereas IL-10 levels increased only 2- to 5-fold, compared to cells stimulated with medium alone. Interestingly, killed H. pylori induced significantly more IL-10 (P < 0.05) than live H. pylori, while recombinant urease only induced IL-10. These results demonstrate that live H. pylori selectively stimulates the induction of IL-12 and Th1 cells that produce IFN-gamma, whereas preparations used in oral vaccines induce more IL-10 and may favor Th2 cell responses.