The present invention provides mutant P-glycoprotein polypeptides that confer increased resistance to certain chemotherapeutic drugs relative to wild-type P-glycoprotein or P-glycoprotein having a glycine to valine substitution at position 185, and nucleic acid molecules encoding the same. The invention also provides antibodies that specifically bind mutant P-glycoproteins. The invention further provides methods for the diagnosis and treatment of conditions associated with P-glycoprotein-mediated multidrug resistance.