We investigate how lipopolysaccharide (LPS) could influence antigen-specific T-cell responses as well as tolerance induction. Using the recall antigen tetanus toxoid for primary in vitro T-cell stimulation, we observed that LPS synergized with peptides to augment proliferation, particularly when used at low concentrations (as little as 100 pg/ml), and that interleukin-12 (IL-12) was partially required for this synergistic effect. Because of the clear enhancement of in vitro peptide-specific responses we then tested whether LPS could influence antigen-specific tolerance driven by coincubation of antigen (tetanus toxoid; TT or immunodominant peptides) with human CTLA-4Ig fusion protein. As expected, CTLA-4Ig treatment inhibited responses to peptides. LPS (100 pg/ml) induced a partial recovery of primary in vitro proliferation under these conditions and the presence of LPS during the primary stimulation prevented the induction of tolerance normally observed on re-stimulation with the same antigen alone. Contrary to the synergistic effects on peptide proliferation this action was not caused by release of IL-12. In addition, the neutralization of tumour necrosis factor-alpha (TNF-alpha) during the primary stimulation did not inhibit proliferation on re-stimulation with peptide. LPS could therefore exert dramatic effects on antigen-specific proliferation and CTLA-4Ig-induced non-responsiveness in human T cells, although via distinct mechanisms. These results reinforce the evidence that LPS influences T-cell function, most likely as a consequence of myeloid cell activation.