The initial discovery of p-glycoprotein in the plasma membrane of MDR cancer cell lines was followed quickly by the cloning of its gene. Sequence analysis of cloned cDNAs revealed that p-glycoprotein was a member of the ABC family of membrane transporters. Subsequent biochemical characterization demonstrated the binding of chemotherapeutic drugs and ATP to p-glycoprotein. P-glycoprotein-mediated drag transport and drug-stimulated ATPase activity were documented in plasma membrane vesicles and in proteoliposomes containing the partially purified protein. P-glycoprotein was shown to be phosphorylated and the effect of this modification on the protein's biological function was explored. P-glycoproteins were found in many normal tissues and their overexpression was documented in numerous cancers. An important role for p-glycoprotein in intrinsic and acquired drug resistance in clinical oncology was established. Despite all that has been learned about p-glycoprotein over the last few years, additional studies will be necessary to address the many questions that have been left unanswered. Determination of p-glycoprotein structure and membrane topology should help elucidate the nature of chemotherapeutic drug binding sites and the mechanism whereby drug movement is coupled to ATP hydrolysis. Complete purification and functional reconstitution of p-glycoprotein into defined lipid vesicles will permit further characterization of drug transport and ATPase activity and give us the means by which p-glycoprotein's apparent dual function as a transporter and a channel can be clarified. Structural and functional studies on p-glycoprotein will also provide information needed to develop specified inhibitors that can be used clinically to overcome MDR in cancer patients. Further study of the mechanisms whereby p-glycoprotein expression is induced and regulated during malignant transformation is indicated. The development of biliary phospholipid deficiency in mdr2 knockout mice and xenobiotic hypersensitivity in mdr3 knockout mice have given us the first clues into the normal physiologic roles for the p-glycoproteins. The search for endogenous substrates for the p-glycoproteins will continue to be an area of active investigation. Continued investigation of p-glycoprotein's functions should result in better understanding of an important class of prokaryotic and eukaryotic membrane transporters. The potential of exploiting the knowledge garnered from these studies in the treatment of neoplastic, parasitic and inherited and acquired liver disease may be greater than we can now imagine.