Meisoindigo has been a routine therapeutic agent in the clinical treatment of chronic myelogenous leukemia (CML) in China since the 1980s. In the present study, the in vitro antileukemic activity of meisoindigo was investigated in acute promyelocytic leukemia (APL) cells, acute myeloid leukemia (AML) cells, and myelomonocytic leukemia cells (NB4, NB4.007/6, HL-60 and U937) comprising both retinoic acid-sensitive and retinoic acid-resistant cells. We found that meisoindigo effectively inhibited the growth and/or proliferation of these four cell types at µM levels. The effects of meisoindigo in these cells are related to its proliferation inhibition and apoptosis induction, and are independent of cell cycle arrest, indicating that meisoindigo could be possible in the treatment of APL, AML and retinoic acid resistant APL. The in vivo pharmacokinetics of meisoindigo and its major circulatory metabolites in rat plasma were then investigated by a newly developed and validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. The profiles of plasma concentration versus time were plotted and the relevant pharmacokinetic parameters were calculated for meisoindigo and its reductive metabolites. The plasma concentrations of meisoindigo after oral administration were much lower than the in vitro IC50s determined in the leukemic cells. The contradicting poor pharmacokinetic characteristics and the established clinical efficacy of meisoindigo could indicate the presence of active metabolites in vivo.