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INTRODUCTION: A key factor in the human adaptive immune response towards Cryptosporidium, associated with resistance to infection, is IFNg. Its function and the cells involved in clearing the parasite from the human intestinal epithelium are unknown. As CD4+ and CD8+ cells produce IFNg after re-stimulation with parasite antigens in vitro we hypothesized that CD8+ cells contribute to control of the infection. METHODS: We studied CD8+ cell cytotoxicity by 51Chromium (51Cr) release assay and fluorescent microscopy. Targets were 51Cr labeled CaCo2 cells. Effectors were CD8+ cells isolated from peripheral blood mononuclear cells of sensitized or naïve volunteers after 6 day culture with IL-2 + IL-15 +/- C. hominis glycoprotein 15 (gp15). RESULTS: Gp15 stimulated CD8+ cells from a sensitized donor matched at HLA-A and HLA-B killed infected cells, with killing increasing with effector numbers, in contrast to CD8+ cells from a naïve donor. In microscopy experiments the same primed CD8+ cells released cytolytic granules into infected cells. CD8+ cells from sensitized donors matched at HLA-A, but not at HLA-B did not kill infected targets as efficiently. HLA-A and HLA-B antibodies inhibited cytotoxicity. CONCLUSION: The data suggest that re-stimulated CD8+ cells from sensitized individuals kill infected cells through release of cytotoxic granules. HLA-B seems to be at least partially required for peptide recognition.