The development of many autoimmune diseases has been etiologically linked to exposure to infectious agents 1. For example, a subset of patients with a history of Salmonella infection develop reactive arthritis 2, 3, 4, 5, 6. The persistence of bacterial antigen in arthritic tissue and the isolation of Salmonella or Yersinia reactive CD8+ T cells from the joints of patients with reactive arthritis support the etiological link between Gram-negative bacterial infection and autoimmune disease 7, 8. Models proposed to account for the link between infection and autoimmunity include inflammation-induced presentation of cryptic self-epitopes, antigen persistence and molecular mimicry 1. Several studies support molecular mimicry as a mechanism for the involvement of class II epitopes in infectious disease-induced self-reactivity 9, 10, 11, 12. Here, we have identified an immunodominant epitope derived from the S. typhimurium GroEL molecule. This epitope is presented by the mouse H2-T23-encoded class Ib molecule Qa-1 and was recognized by CD8+ cytotoxic T lymphocytes induced after natural infection. S. typhimurium-stimulated cytotoxic T lymphocytes recognizing the GroEL epitope cross-reacted with a peptide derived from mouse heat shock protein 60 and recognized stressed macrophages. Our results indicate involvement of MHC class Ib molecules in infection-induced autoimmune recognition and indicate a mechanism for the etiological link between Gram-negative bacterial infection and autoimmunity.