The cell line COR-L23/R is a multi-drug resistant (MDR) sub-line derived from the parent line COR-L23 after treatment with doxorubicin (adriamycin). The P-glycoprotein negative cell line is cross resistant to daunorubicin, vincristine and rhodamine 123. Drug resistance is caused by enhanced drug efflux that can be inhibited by addition of buthionine sulphoximide (BSO), an inhibitor of glutathione synthesis. COR-L23/R contain reduced levels of glutathione and glutathione-S-transferase activity compared to the parent line. MDR seems to be connected with overexpression of multi-drug resistance-associated protein (MRP) which is discussed as a drug transporter.