Inherent or acquired resistance to multiple natural product drugs in human tumour cells is often associated with increased expression of multidrug resistance protein (MRP), a 190-kDa integral membrane protein that belongs to the ATP-binding cassette (ABC) superfamily of transport proteins. Both clinical and experimental investigations of MRP have been facilitated by several monoclonal antibodies (MAbs) generated against intracellular epitopes of the molecule. Recently, however, several new ABC transporters that are quite closely related to MRP have been identified, raising concerns about the specificity of the MRP-reactive MAbs. In the present study, we have mapped the epitopes of MAbs MRPr1 and MRPm6 to the decapeptides 238GSDLWSLNKE247 (located in the intracellular loop between the first and second membrane-spanning domains of MRP) and 1511PSDLLQQRGL1520 (located near the carboxy terminus of MRP) respectively. Alignment of the MRPr1 and MRPm6 epitope sequences with the comparable regions in mammalian ABC proteins most closely related to MRP indicates that, with the exception of murine mrp, the sequences are poorly conserved. We conclude that MAbs MRPm6 and MRPr1, together with MAb QCRL-1, which has previously been mapped to the heptapeptide 918SSYSGDI924, remain highly specific probes for detection of different regions of the MRP molecule.