Multidrug resistance involves cross‐resistance between some of the most important agents in cancer chemotherapy, including the anthracyclines and the vinca alkaloids. It is closely related to an active extrusion of the drugs from the resistant cells, which is related to an overexpression of the multidrug resistance gene encoding a membrane protein, the P‐glycoprotein, that has homologies with bacterial transport proteins. The extrusion mechanism is accessible to inhibition by several means, and particularly the use of competitive inhibition with non‐toxic analogues of the drugs has proved to be an effective way to reverse multidrug resistance in animal tumorsin