The induction of tumor cell differentiation represents an attractive strategy for the treatment of a wide range of malignancies. Differentiation of HL-60 promyelocytic leukemia cells towards neutrophils or monocytes has been shown to induce apoptotic cell death, which is inhibited by bcl-2 over-expression. However, the role of the bcl-2 gene family during erythroid differentiation of human leukemia cells remains unknown. We found that human erythroleukemia (HEL) and K562, two leukemia cell lines that undergo erythroid differentiation do not express Bcl-2, but express Bcl-XL, a related protein that functions as an inhibitor of apoptosis. Differentiation of HEL or K562 cells with inducers of erythroid differentiation (hemin, retinoic acid, or transforming growth factor-beta) was accompanied by progressive cell death and degradation of genomic DNA into oligonucleosomal fragments. The loss of cellular viability was associated with downregulation of bcl-xL mRNA and protein. In contrast, the levels of Bax, another Bcl-2 family member implicated in apoptosis remained unaltered. Constitutive expression of Bcl-XL by gene transfer inhibited apoptosis triggered by erythroid differentiation of HEL K562 cells. Yet, Bcl-XL did not alter the expression of epsilon-globin, which is induced during erythoid differentiation of HEL and K562 cells, arguing that apoptosis and differentiation can be uncoupled by Bcl-XL. These results indicate that Bcl-XL acts as an antiapoptosis protein in leukemia cells that undergo erythroid differentiation and that downregulation of bcl-x is a component of the apoptotic response that is coupled to differentiation in human leukemia cells.