A whole inactivated H. pylori bacterium preparation was found to stimulate blood mononuclear cells from both antibody-positive and antibody-negative subjects, but the antibody-positive subjects tended to have lower proliferation responses. The present study was designed to characterize T cell activation further by measuring several components of the response. Eighty-seven subjects (80 dyspeptic patients and seven healthy persons from the laboratory staff) with or without antibodies to H. pylori were studied by measuring the DNA synthesis induced by several H. pylori concentrations (1-23 micrograms/ml) and the control stimulants PPD, tetanus toxoid and pokeweed mitogen (PWM). H. pylori-induced secretion of interleukin-2 (IL-2), tumour necrosis factor-alpha (TNF-alpha), interleukin-4 (IL-4), soluble CD8 and IL-2 receptor (IL-2R) molecules and H. pylori- and PPD-induced appearances of IL-2R+ and HLA-DR+ T cells were measured in a smaller number of subjects. H. pylori-induced DNA synthesis was again lower in the antibody/bacterium-positive subjects, while no differences between the two groups were found in cultures stimulated by unrelated antigens or PWM. Soluble IL-2R and TNF-alpha were detectable in cultures with H. pylori from all subjects, while the amount of IL-2 did not differ from that in the background culture. No differences were found in the amounts of IL-2 or soluble IL-2R between the antibody-positive and negative subjects; while the former tended to secrete more soluble CD8 molecules, a difference which was significant with the smaller H. pylori concentration used (P less than 0.01). The numbers of HLA-DR+ and IL-2R+ T cells increased in cultures with H. pylori or PPD from all the subjects, the majority of both cells having the CD4 phenotype. Numbers of DR+ and IL-2R+ T cells were similar in the cultures of the antibody-positive and negative subjects, but the respective CD8 subsets were increased in the former. The confirmed decrease in proliferation in the antibody-positive subjects does not seem to be connected with lower IL-2/IL-2R responses but may involve CD8 cell activation.