Abstract: The invention relates to a method for testing L0-specific inhibitors of transporter proteins of the MRP-family and for testing inhibitors specific to the predicted amphipatic helical region of the L0 region, the tested inhibitors L0 peptide, mutants thereof, polynucleotides coding the peptides. Vectors and hosts cells comprising the polynucleotides and reagent kits for testing and developing the inhibitors. The invention is especially useful for combatting multidrug resistance in cancer patients and in the field of related research.