Acute promyelocytic leukemia (APL) is characterized by the t(15;17) which involves the PML gene and the retinoic acid receptor alpha (RAR alpha) gene, and the subsequent PML/RAR alpha fusion gene is a key event in the leukemogenesis of APL. We found that the PML/RAR alpha fusion gene was expressed in both granulocytic/macrophage and erythroid colonies in a few patients with APL. In some instances of acute myelogenous leukemia (AML), erythrocytes or platelets also expressed the glucose-6-phosphate dehydrogenase (G-6-PD) isoenzymes which were detected in the leukemic cells. Some APL cells show basophilic and monocytoid differentiation and these findings suggest that the leukemic precursor of APL is derived from a more primitive cell stage than the promyelocyte. The precursor cells appear to be derived from heterogeneous levels.