The expression of the proto-oncogene c-myc was studied at the protein level in cells obtained from patients with AML and CML. In florid AML and during the blastic phase of CML the majority of cells contain c-myc protein with the amount of protein differing widely among the cells of individual patients. In contrast, during complete remission in AML and during the chronic phase of CML cells containing c-myc protein are rare. Several studies demonstrated a discordance in the amount of c-myc transcript and the amount of c-myc protein present in cell populations thereby suggesting the presence of translational or post-translational regulation of c-myc expression. Further, the data suggest that high levels of c-myc protein in the leukemic cells of AML patients are associated with a poor response to therapy and that high levels in AML patients in CR or in the peripheral blood of chronic phase CML patients may be indicative of impending acute leukemia.