Using recombinant DNA technology, a transgenic mouse has been engineered whose bone marrow is protected from the toxic effects of chemotherapy by expression of the MDR 1 gene. This 'animal system' allows rapid screening of drugs which inhibit the multidrug transporter and heralds a new era of using transgenic animals for pharmacologic screening. Multidrug resistance resulting from expression of an energy-dependent drug efflux pump encoded by the human MDR gene is a major impediment to effective