Bcr-Abl tyrosine kinase activity initiates a number of intracellular signaling cascades that result in leukemogenesis. Imatinib mesylate, a specific Bcr-Abl tyrosine kinase inhibitor, has been highly successful in the treatment of chronic myelogenous leukemia (CML). However, the emergence of imatinib resistance and the incomplete molecular response of a significant number of patients receiving this therapy have led to a search for combinations of drugs that will enhance the efficacy of imatinib. We have demonstrated that mycophenolic acid (MPA), a specific inosine monophosphate dehydrogenase (IMPDH) inhibitor that results in depletion of intracellular guanine nucleotides, is synergistic with imatinib in inducing apoptosis in Bcr-Abl-expressing cell lines. Studies of signaling pathways downstream of Bcr-Abl demonstrated that the addition of MPA to imatinib reduced the phosphorylation of both Stat5 and Lyn, a Src kinase family member. The phosphorylation of S6 ribosomal protein was also greatly reduced. These results demonstrate that inhibitors of guanine nucleotide biosynthesis may synergize with imatinib in reducing the levels of minimal residual disease in CML and lay the foundation for clinical trials in which IMPDH inhibitors are added to imatinib in patients who have suboptimal molecular responses to single agent therapy or who have progressive disease.