IL-1 is considered the primary mediator of the acute phase response. One of the characteristic manifestations of this response is early neutrophilia that is probably caused by release of mature neutrophils from the bone marrow into the peripheral blood. In the present study, we assessed whether IL-1 had a similar releasing effect on the number of circulating progenitor cells and stem cells. Female BALB/c mice were injected i.p. with increasing (0.1-1.0 micrograms/mouse) concentrations of rhu-IL-1 alpha. IL-1 injection resulted in a marked dose-dependent increase in the number of polymorphonuclear neutrophils, granulocyte-macrophage colony-forming units (CFU-GM), and cells forming spleen colonies (CFU-S day 8 and day 12). The maximal increase was found at 4 to 8 h after injection of 1 micrograms IL-1 per mouse, yielding a mean fivefold elevation in neutrophil count, and a mean 30-fold and 10-fold increase in the number of circulating CFU-GM and CFU-S, respectively. In a subsequent series of experiments, lethally irradiated (8.5 Gy) female recipient animals were transplanted with 5 x 10(5) blood mononuclear cells derived from male IL-1-treated animals. Long-term survival was obtained in 68% of mice transplanted with peripheral blood cells derived from donor animals at 6 h after a single injection of 1 micrograms IL-1. The mean number of circulating CFU-GM in these donor animals was 557/ml blood. At 6 mo after transplantation, greater than 95% of the bone marrow cells were of male origin, as determined using in situ hybridization with a Y-chromosome specific probe. In contrast, long-term survival was reached in less than 10% of mice transplanted with an equal number of blood cells derived from saline-treated controls or donor animals treated with a dose of 0.1 micrograms IL-1. These results indicate that a single injection of IL-1 induces a shift of hematopoietic progenitor cells and marrow repopulating cells into peripheral blood and that these cells can be used to rescue and permanently repopulate the bone marrow of lethally irradiated recipients.