The multidrug-resistance phenotype expressed in mammalian cell lines is complex. Cells selected with a single agent can acquire cross-resistance to a remarkably wide range of compounds which have no obvious structural or functional similarities. The basis for cross-resistance seems to be a decreased net cellular accumulation of the drug involved, and has been attributed to alterations in the plasma membrane. An over-expressed plasma membrane glycoprotein of relative molecular mass (Mr) 170,000 (P-glycoprotein) is consistently found in different multidrug-resistant human and animal cell lines, and in transplantable tumours. Consequently, it has been postulated that P-glycoprotein directly or indirectly mediates multidrug resistance. Here we report the cloning of a complementary DNA encoding P-glycoprotein. Southern blot analysis of hamster, mouse and human DNA using this cDNA as a probe showed that P-glycoprotein is conserved and is probably encoded by a gene family, and that members of this putative family are amplified in multidrug-resistant cells.