Drug-resistant cancer cells with the multidrug-resistance phenotype show overexpression of P-glycoprotein, and we therefore tested carcinoma tissue from five patients with stage III or IV ovarian cancer for P-glycoprotein using 265/F4 and C 219 monoclonal antibodies, prepared against membrane glycoproteins in colchicine-resistant CHO cells. Using immunofluorescence and immunoblotting techniques, one of the tumors showed a positive reaction. Using the pcDR 1.5 clone we found that the same cancer tissue had elevated expression of the genes responsible for multidrug resistance. The demonstration of elevated P-glycoprotein in ovarian carcinomas indicates that P-glycoprotein overexpression is not limited to experimental tumor models.