Imatinib is a tyrosine kinase inhibitor that is effective in the treatment of chronic myeloid leukemia (CML). Not all patients achieve cytogenetic response. Some patients even lose the initial cytogenetic response. In this study, we investigated the active cellular transport of imatinib to gain a better understanding of the possible mechanisms of imatinib resistance. We used the leukemic cell line CCRFCEM and its drug-resistant subline VBL(100) to measure the uptake of carbon 14 ((14)C)-labeled imatinib. Imatinib uptake was temperature dependent, indicative of an active uptake process. Additionally, incubations with transport inhibitors showed that verapamil, amantadine, and procainamide, inhibitors of the human organic cation transporter 1 (hOCT1), significantly decreased imatinib uptake into CEM cells, whereas the inhibition of hOCT2 or hOCT3 had no effect, indicating that influx into the cells is an active process likely to be mediated by hOCT1. Studies using transfected MDCK cell lines revealed an active efflux component attributable to MDR1 (ABCB1). Both hOCT1 and MDR1 were expressed in CML primary cells and cell lines. The results indicate that active transport processes mediate the influx and efflux of imatinib. Differential expression of influx (hOCT1) and efflux (MDR1) transporters may be a critical determinant of intracellular drug levels and, hence, resistance to imatinib.