Three monoclonal antibodies (mAb), MRK16, MM4.17 and MC57, directed against distinct epitopes on the external domain of human P-glycoprotein (Pgp), were used to follow its expression on multidrug resistant (MDR)-cells. The linear MM4.17 epitope and conformational MRK16 epitope showed a 4-fold higher expression at 37 degrees C than at 4 degrees C, while the detection of the conformational MC57 epitope did not change. Inhibition of Pgp function, by a short pretreatment of the MDR-cells with resistance-modulating agents (RMA), such as SDZ PSC 833 and SDZ 280-446, could not be related to depletion of Pgp from the cell surface, since their expression of the MM4.17 and MRK16 epitopes was found unchanged. However, a substantially higher expression of MC57 epitopes was found on RMA-treated cells than on untreated ones. Since this effect correlated to the strength of different RMA in reversing the MDR phenotype, MC57 epitopes might be more efficiently expressed on inactivate(d) forms of the Pgp molecules, suggesting that RMA might inhibit Pgp function by disturbing the conformation of individual Pgp molecules, their topographical distribution or polymerization status in the membrane.