The present work was performed to elucidate the active moiety of a sesquiterpene lactone, taraxinic acid-1′-O-β-D-glucopyranoside (1) from Taraxacum coreanum NAKAI on the cytotoxicity of various cancer cells. Based on enzymatic hydrolysis and MTT assay, the active moiety should be attributed to the aglycone taraxinic acid (1a), rather than the glycoside (1). Taraxinic acid exhibited potent antiproliferative activity against human leukemia-derived HL-60. In addition, this compound was found to be a potent inducer of HL-60 cell differentiation as assessed by a nitroblue tetrazolium reduction test, esterase activity assay, phagocytic activity assay, morphology change, and expression of CD14 and CD66b surface antigens. These results suggest that taraxinic acid induces the differentiation of human leukemia cells to monocyte/macrophage lineage. Moreover, the expression level of c-myc was down-regulated during taraxinic acid-dependent HL-60 cell differentiation, whereas p21^CIP1 and p27^KIP1 were up-regulated. Taken together, our results suggest that taraxinic acid may have potential as a therapeutic agent in human leukemia.