Exposure of acute promyelocytic leukemia (APL) cells to all-trans retinoic acid (ATRA) increases levels of Mcl-1, however, the implication of ATRA-mediated expressions of Mcl-1 in these cells remains to be fully elucidated. This study found that exposure of NB4 and PL-21 cells to ATRA increased levels of Mcl-1 in association with phosphorylation of c-jun N-terminus kinases. Down-regulation of Mcl-1 by a small interfering (siRNA) or an inhibitor of JNK significantly potentiated the ability of ATRA to induce differentiation and apoptosis in these cells. On the other hand, the anti-leukemia effects of ATRA were blunted when Mcl-1 was forced expressed in NB4 and PL-21 cells as well as leukemia cells isolated from individuals with APL. Furthermore, down-regulation of Mcl-1 by an siRNA sensitized non-APL U937 and KG-1 leukemia cells to ATRA-mediated differentiation and apoptosis. Taken together, inhibition of Mcl-1 might be useful to potentiate the action of ATRA in APL as well as non-APL AML cells.