Although IL-12 has been reported to synergize with c-kit ligand (KL) in promoting hematopoietic stem cell proliferation in vitro, administration of recombinant mouse IL-12 (rIL-12) to normal mice caused a dose- and time-dependent anemia, leukopenia, and thrombocytopenia in vivo. Decreased numbers of bone marrow cells were recovered from the tibiae of IL-12-treated mice, and histologic examination of the marrow revealed a loss of mature neutrophils and red blood cell precursors. However, simultaneously with the suppression of hematopoiesis in the bone marrow, the IL-12-treated mice developed splenomegaly, which was largely caused by a marked enhancement of splenic extramedullary hematopoiesis of the erythroid, myeloid, and megakaryocytic lineages. These histologic observations were confirmed by colony-forming cell assays in which administration of IL-12 was shown to cause a time-dependent decrease in bone marrow CFU-GM, CFU-E, and BFU-E hematopoietic colony-forming cells while causing an increase in splenic CFU-GM and BFU-E colony-forming cells. All these effects were reversible upon cessation of IL-12 treatment. The observation that in IL-12-treated mice hematopoiesis was suppressed in the marrow but enhanced in the spleen suggests that myelosuppression was not caused by a direct effect of IL-12 on hematopoietic progenitors. It seems likely that myelosuppression was caused instead by an IL-12-induced alteration in the local environment of the marrow.