Bacteria of the genus Yersinia cause disease in humans ranging from enteritis and lymphadenitis to plaque. These organisms have the ability to bind to host cells and, in a complex interaction, are able to deliver a specific subset of proteins into the host cell. These proteins alter host structures and functions, and in the case of macrophages, prevent phagocytosis of the organism. In Yersinia enterocolitica, one of these proteins, Yop51, is known to be a protein tyrosine phosphatase. By using a directed approach, we have raised murine CTL that recognize a peptide epitope from Yop51 in the context of the MHC class I molecule H-2 Db. This epitope, Yop51 249-257, seems to be presented both by cells that produce Yop51 endogenously and by epithelial cells infected with virulent Y. enterocolitica. In vivo, Y. enterocolitica is believed to remain mostly in an extracellular niche. Therefore, detection of this organism by cytotoxic T cells and, possibly, resistance to disease may depend on recognition of epitopes from a subset of virulence determinants delivered into the cytoplasm of host cells by surface-bound organisms.