Objective. Pluripotent hematopoietic stem cells and muscle-derived hematopoietic potential cells isolated by Hoechst 33342 dye-mediated fluorescein-activated cell sorting (FACS) as side population (SP) cells, give rise to hematopoietic cells as well as skeletal muscle cells following intravenous transplantation. However, besides bone marrow and skeletal muscle, it has remained unclear whether other adult tissues also contain SP cells that are enriched for cells that exhibit hematopoietic potential. Methods. To test whether adult tissues contain SP cells with hematopoietic potential, Hoechst-FACS analysis and hematopoietic colony formation assays were performed with cells isolated from a variety of adult tissues, skeletal muscle, heart, brain, spleen, liver, kidney, lung, and small intestine and compared with peripheral blood and bone marrow cells. Results. In addition to hematopoietic tissues, cell preparations from nonhematopoietic tissues, such as skeletal muscle, kidney, lung, and small intestine, displayed markedly higher hematopoietic colony formation activity compared to peripheral blood cells. Moreover, the hematopoietic progenitors in these adult tissues expressed the hematopoietic cell marker CD45. Hoechst-FACS analysis demonstrated that all adult tissues examined contained SP cells. In addition, these SP fractions were enriched for cells that efficiently formed hematopoietic colonies in vitro. Conclusion. These results indicate that hematopoietic progenitors are present in significant numbers in all adult tissues examined.