We examined the effect of daunorubicin (DNR), the new anthracycline derivative idarubicin (IDR), and verapamil on two leukemia cell lines that displayed the multidrug resistant (MDR) phenotype and used laser flow cytometry to quantitate intracellular anthracycline content. The vinblastine-resistant human lymphoblastic leukemia cell line CEM-VBL demonstrated minimal DNR uptake; simultaneous incubation with verapamil and DNR increased intracellular DNR uptake fourfold. IDR uptake was 10 times more rapid in these cells and simultaneous incubation with IDR and verapamil resulted in only a 1.2-fold increase of intracellular IDR. Similar results were observed in the vincristine-resistant human myeloid leukemia cell line HL-60/RV+. Intracellular retention of DNR and IDR was also measured in each cell line. In CEM-BVL cells, 38% of the original DNR concentration remained after a 2-hour resuspension in fresh medium compared with 71% of the original IDR concentration. In HL-60/RV+ cells, 36% of the DNR concentration remained compared with 51% of the IDR concentration. After incubation of CEM-VBL and HL-60/RV+ cells with DNR for 1 hour followed by resuspension in fresh medium plus verapamil, intracellular DNA retention increased 5- and 5.2-fold, respectively. However, incubation of these cells for 1 hour with IDR followed by resuspension in fresh medium plus verapamil resulted in only a 1.6- and 2.4-fold increase in intracellular IDR retention. Lastly, clonogenic experiments were performed to correlate intracellular anthracycline content with cytotoxicity. DNR alone had a minimal effect on the clonogenic growth of CEM-VBL cells, whereas the combination of DNR plus verapamil resulted in approximately 80% growth inhibition. However, incubation of these cells with IDR alone resulted in greater than 95% growth inhibition. These results suggest that IDR may be more effective than DNR in leukemia cells that display the MDR phenotype.