Interleukin-8 (IL-8) belongs to a family of chemoattractant cytokines involved in chemotaxis and activation of neutrophils. As in vivo administration of IL-8 induces granulocytosis and the release of immature white blood cells into the circulation, we assessed a possible mobilizing effect of IL-8 on myeloid progenitor cells. IL-8 was administered at intraperitoneal doses ranging from 0.1 to 100 micrograms per mouse to female Balb/C mice (aged 8 to 12 weeks; weight, 20 to 25 g). Animals were killed at time intervals ranging from 1 to 240 minutes after IL-8 administration, and blood, bone marrow, and spleen cells were harvested. Injection of 30 micrograms IL-8 resulted in an increment from 25 +/- 9 to 418 +/- 299 granulocyte-macrophage colony-forming units (CFU-GM) per milliliter blood at 15 minutes after a single intraperitoneal injection. Sixty minutes after the injection of IL-8, the numbers of circulating CFU-GM per milliliter blood had almost returned to pretreatment values (82 +/- 39 CFU-GM per milliliter). A dose of 100 micrograms IL-8 per animal did not result in a further increment in the number of circulating CFU-GM. Transplantation of 5 x 10(5) blood-derived mononuclear cells (MNC) obtained at 30 minutes after IL-8 injection (30 micrograms) resulted in 69% survival of lethally irradiated (8.5 Gy) recipients at 60 days versus 22% for animals transplanted with an equal number of nonprimed blood-derived MNC. Transplantation of 1.5 x 10(6) MNC obtained from IL-8-treated donors resulted in 100% survival. Six months after transplantation, female recipients of MNC derived from IL-8-treated male donors were killed, and chimerism was determined in bone marrow, spleen, and thymus using a Y chromosome-specific probe and fluorescent in situ hybridization (FISH). The majority of bone marrow, spleen, and thymus cells (83% +/- 25%, 89% +/- 5%, and 64 +/- 28%, respectively) consisted of Y chromosome-positive cells, showing that the IL-8-mobilized cells had myelolymphoid repopulating ability. We conclude that IL-8 is a cytokine that induces rapid mobilization of progenitor cells and pluripotent stem cells that are able to rescue lethally irradiated mice and that are able to completely and permanently repopulate host hematopoietic tissues.