Although differentiation therapy for patients with acute promyelocytic leukemia (APL) using all- trans retinoic acid (ATRA) has now been established, acute myeloid leukemia (AML) patients with other than APL only show a limited clinical response to ATRA. We must consider novel therapeutic drugs against other AML to develop a differentiation therapy for leukemia. Regulators that play an important role in the differentiation and development of plants may also affect the differentiation of human leukemia cells through a common signal transduction system, and might be clinically useful for treating AML. Cytokinins are important purine derivatives that serve as hormones that control many processes in plants. Cytokinins such as kinetin, isopentenyladenine (IPA) and benzyladenine were very effective at inducing nitroblue tetrazolium (NBT) reduction and morphological changes in human myeloid leukemia cells into mature granulocytes. On the other hand, cytokinin ribosides such as kinetin riboside, isopentenyladenosine (IPAR) and benzyladenine riboside were the most potent for inhibiting growth and inducing apoptosis. When the cells were incubated with cytokinin ribosides in the presence of an O 2 m scavenger, antioxidant or caspase inhibitor, apoptosis was significantly reduced and differentiation was greatly enhanced. These results suggest that both cytokinins and cytokinin ribosides can induce the granulocytic differentiation of HL-60 cells, but cytokinin ribosides also induce apoptosis prior to differentiation. Cotylenin A has been isolated as a plant growth regulator exhibits cytokinin-like activity. Although it has a different structure than cytokinins, it also induces the differentiation of human myeloid leukemia cells. These results suggest that there is an association between the action of plant redifferentiation-inducing hormones and the mechanism of the differentiation of human leukemia cells.