In the present study, we aimed to clarify the capacity of human cord blood- and bone marrow-derived progenitor cells to generate gastrointestinal epithelial cells in clinical and experimental transplantation settings. First, in a clinical transplantation setting, gastrointestinal tissues derived from female pediatric or juvenile recipients of allogeneic sex-mismatched bone marrow and cord blood transplantation were examined for the presence of donor-derived epithelial cells. Gastrointestinal specimens of allogeneic recipients included Y chromosome+ cytokeratin+ epithelial cells at a frequency of 0.4-1.9%. To further determine the capacity of purified human progenitor cells, human cord blood- or bone marrow-derived CD34+ cells were transplanted into newborn NOD/SCID/beta2-microglobulin(null) mice as an experimental transplantation assay. When gastrointestinal tissues derived from recipient mice were subjected to FISH and immunofluorescence analyses, human epithelial cells were identified at a frequency of 0.24-0.58% at 3 months posttransplantation. Finally, double FISH analyses using species-specific probes revealed that human chromosome+ epithelial cells did not possess any murine chromosomes, indicating that donor-derived epithelial cells were not generated only by cell fusion. On the basis of these findings, it is concluded that purified human cord blood and bone marrow CD34+ progenitor cells can generate gastrointestinal epithelial cells across allogeneic and xenogeneic histocompatibility barriers.