Multidrug resistance represents a major obstacle in the successful therapy of neoplastic diseases. Studies have demonstrated that this form of drug resistance occurs both in cultured tumor cell lines as well as in human cancers. P-glycoprotein appears to play an important role in such cells by acting as an energy-dependent efflux pump to remove various natural product drugs from the cell before they have a chance to exert their cytotoxic effects. Expression of the MDR1 gene product has been associated with a poor prognosis in clinical studies. It has been demonstrated in the laboratory that resistance mediated by the P-glycoprotein may be modulated by a wide variety of compounds. These compounds, which include verapamil and cyclosporin, generally have little or no effect by themselves on the tumor cells, but when used in conjunction with antineoplastic agents, they decrease, and in some instances eliminate, drug resistance. Clinical trials to modulate P-glycoprotein activity are underway at the present time to determine if such strategies will be feasible. Although the P-glycoprotein is expressed in many cell lines and occurs in patient tumors, its expression is not a universal feature of multidrug resistance, suggesting that other mechanisms are operating.