After a 1-hour incubation with rhodamine, cells were exposed for 2 hours to dye-free medium without (1) or with (2) verapamil to allow rhodamine efflux. Distribution of rhodamine content of normal CD34⁺ cells (A) and AML blasts (B and C) was measured by flow cytometry. Efflux inhibition by verapamil (2) resulted in an increase of rhodamine content in cells with MDR expression.