1. Multidrug resistance (MDR) is a phenomenon originally seen in cultured tumor cells that, following selection for resistance to a single anticancer agent, become resistant to a range of chemically diverse anticancer agents. These MDR cells show a decrease in intracellular drug accumulation due to active efflux by transporter proteins. The transporter best characterized is P-glycoprotein (Pgp). This protein has been identified in many cancers and has been the target for agents able to inhibit its action, thereby reversing resistance. 2. More recently, another transporter, multidrug resistance-associated protein (MRP) has been identified in a number of MDR human tumor cell lines that do not apparently express Pgp. The presence of MRP at the cell surface of these cells is associated with alterations in drug accumulation and distribution. 3. The gene-encoding MRP has been cloned and sequenced and shown by transfection studies to be able to confer resistance and changes in drug accumulation in sensitive tumor cells. The profile of anticancer drugs expelled in the presence of MRP is similar, but not identical, to that of Pgp. 4. MRP has been identified in a number of different types of cancers, but it is not yet clear to what extent it is involved with clinical resistance. Furthermore, resistance modulators useful against Pgp are less effective in reversing MRP-mediated resistance. 5. It is not fully understood how MRP brings about drug efflux, but it is clear that the underlying mechanisms are different from those responsible for Pgp-mediated drug efflux. In particular, glutathione (GSH) is required for the effective expulsion of the anticancer agents. 6. Unlike Pgp, MRP is able to transport metallic oxyanions and glutathione and other conjugates, including peptidyl leukotrienes. Agents that inhibit organic anion transport, such as probenecid, can block MRP activity. 7. Like Pgp, MRP is expressed not only in resistant tumor cells, but also in normal human tissues. These include the epithelial cells lining the airways and the gastrointestinal tract. In cells in normal tissues, MRP appears to be located within the cytoplasm, which may mean that it functions here in a manner slightly different to that in malignant cells. It is now also recognized in cells and tissues from other species, such as the rat and mouse.