In this study, hematopoietic cells from human placental blood were treated with the differentiation-inducing drug all-trans retinoic acid (ATRA). Their cytoplasm was then used to culture the human myelogenous leukemia cell line HL60, NB₄ cells and the RA-resistant HL60-R and NB₄-R₂ cells. All of these four kinds of leukemia cells underwent macrophage/monocyte differentiation and apoptosis, while their proliferation was inhibited. This suggests that the RA receptors were not essentially needed in this situation, and this method should be applicable in the treatment of RA-resistant promyelocytic leukemia as well as other kinds of leukemia.