Summary—Flow cytometry has recently become a very important tool in the study of the mechanisms of multidrug resistance (MDR). This technique allows rapid access to information regarding two characteristics related to the MDR phenotype: i) the level of overexpression of the P‐glycoprotein; and ii) its functional aspect in expulsion of the cytotoxic agents to which the cell is exposed. In pharmacology, flow cytometry also allows evaluation of the capacity of modulating agents to reverse this resistance, and contributes a deeper insight into the mechanism of action of new molecu