MRP is a M_r 190,000 integral membrane phosphoglycoprotein that is overexpressed in some drug-selected resistant cell lines and has been shown to cause multidrug resistance in transfected cells. Five murine hybridoma cell lines (QCRL-1, QCRL-2, QCRL-3, QCRL-4, and QCRL-6) have been generated which secrete monoclonal antibodies (MAbs) that react specifically with membrane proteins of MRP-overexpressing, multidrug-resistant, drug-selected H69AR cells and MRP-transfected HeLa cells (T5) but not the respective parental (H69) and vector-transfected (C1) cells. The ability of three of these MAbs (QCRL-1, QCRL-2, and QCRL-3) to selectively immunoprecipitate a M_r 190,000 protein from ³⁵S-labeled H69AR and T5 membranes indicates that these MAbs are specific for MRP. MAb QCRL-1 is also capable of detecting the low levels of MRP present in revertant H69PR cells by immunoblot analysis. Indirect immunofluorescence analyses show that MAbs QCRL-1, QCRL-2, and QCRL-3 strongly and differentially react with fixed T5 and H69AR cells but not with unfixed cells, suggesting that these MAbs recognize intracellular MRP epitopes. The availability of reagents for the specific and sensitive immunodetection of MRP should greatly facilitate biological and clinical studies of this novel drug resistance protein.