P-Glycoprotein is an integral membrane protein which mediates the energy-dependent efflux of various antitumor agents from multidrug-resistant cancer cells. Surface plasmon resonance was used for the detection of P-glycoprotein after solubilization from drug-resistant and drug-sensitive Chinese hamster ovary cells and for the analysis of its interaction with cyclosporin A, a competitive inhibitor of drug efflux. Detection of P-glycoprotein relied on its binding to the monoclonal antibody C219 which was immobilized on a sensor chip. Binding of Zwittergent 3-14-solubilized P-glycoprotein to the antibody was concentration-dependent and reflected the relative abundance of P-glycoprotein in both cell lines. It was abolished when C219 was omitted or replaced by a rabbit anti-mouse IgG antibody and considerably reduced after precipitation of P-glycoprotein with wheat germ agglutinin. Preincubation of solubilized proteins with cyclosporin A increased the amount of protein bound to the antibody by approximately 30%. These results indicate that surface plasmon resonance is well suited to the detection of P-glycoprotein from biological samples and shows promise as a tool for the study of its interaction with different drugs.