Multidrug resistance describes an experimental observation which appears to explain cross-resistance to certain structurally unrelated cytotoxic agents, including anthracyclines, vinca alkaloids and podophyllotoxins. It is now clear that a major factor responsible for its development is increased expression of a membrane glycoprotein--P-glycoprotein, which functions as an energy-dependent efflux pump. Recent data, particularly in haematological malignancies such as acute non-lymphocytic leukaemia, myeloma and non-Hodgkin's lymphoma, indicate that P-glycoprotein may be involved in the development of clinical drug resistance. The potential therefore exists for new therapeutic studies aimed at circumventing resistance which develops through this mechanism, by using modulators, such as verapamil, quinidine and several others, which prevent cellular drug efflux by competitive binding to P-glycoprotein.