While mononuclear cells isolated from murine skeletal muscle were shown to be capable of hematopoietic activity, similar hematopoietic cells (HC) recently were reported to exist in primate muscle. We investigated muscle-derived HC from young and adult human subjects. Although hematopoietic stem cells were rare in muscle, their frequency nonetheless was approximately four times greater than in peripheral blood. These cells in muscle appeared to originate from CD45(+) bone marrow cells. Our results suggested an additional function of human skeletal muscle as a reservoir of HC.