Oncogenic RAS alleles encode proteins that accumulate in the guanosine triphosphate (GTP)-bound state. Because post-translational processing of Ras by farnesyltransferase is essential for biologic function, inhibitors of this enzyme have been developed as rational cancer therapeutics. We have investigated farnesyltransferase inhibitor (FTI) L-744,832 in an in vivo murine model of myeloid leukemia that is associated with inactivation of the Nf1 tumor suppressor gene.Nf1 encodes a GTPase activating protein for Ras, andNf1-deficient (Nf1−/−) hematopoietic cells show hyperactive Ras signaling through the mitogen-activated protein (MAP) kinase pathway. L-744,832 inhibited H-Ras prenylation in cell lines and in primary hematopoietic cells and abrogated the in vitro growth of myeloid progenitor colonies in response to granulocyte-macrophage colony-stimulating factor (GM-CSF). This FTI also partially blocked GM-CSF–induced MAP kinase activation, but did not reduce constitutively elevated levels of MAP kinase activity in primaryNf1−/− cells. Injection of a single dose of 40 or 80 mg/kg of L-744,832 increased the amount of unprocessed H-Ras in bone marrow cells, but had no detectable effect on N-Ras. Adoptive transfer ofNf1−/− hematopoietic cells into irradiated mice induces a myeloproliferative disorder that did not respond to L-744,832 treatment. We speculate that the lack of efficacy in this model is due to the resistance of N-Ras and K-Ras processing to inhibition by this FTI.