The Dubin-Johnson syndrome is characterized by an inherited defect in the secretion of amphiphilic anionic conjugates from hepatocytes into the bile. We have recently identified the membrane protein mediating the adenosine triphosphate (ATP)-dependent transport of glutathione and glucuronate conjugates as a multidrug-resistance protein (MRP) and localized it to the canalicular as well as to the lateral hepatocyte plasma membrane. In the present study we show the selective absence of the canalicular isoform of MRP (cMRP) from the hepatocytes in a patient with Dubin-Johnson syndrome by double-label immunofluorescence and confocal laser scanning microscopy using antibodies directed against MRP and dipeptidyl-peptidase IV (DPPIV). Another isoform of MRP was detected, however, in the lateral hepatocyte membrane of the patient. Moreover, MRP was present on immunoblots of erythrocyte membranes from Dubin-Johnson syndrome and normal humans. These findings are analogous to our recent observations on the localization of the rat homolog of MRP and its canalicular isoform, cMrp, in normal and transport-deficient GY/TR- Wistar rat liver. The elucidation of the selective absence of an isoform of MRP and from the canalicular membrane domain in conjunction with the defined substrate specificity of the MRP and cMRP gene-encoded conjugate export pumps contributes to the molecular definition of the transport defect in Dubin-Johnson syndrome.