ATP-binding cassette (ABC) transporters generally contain a number of transmembrane helices. The present invention provides synthetic peptides derived from these transmembrane helices. The peptides inhibit ABC transporter function, presumably by disrupting the structure of the ABC transporter. Negatively charged residues are added at the extracellular terminus to promote correct orientation of the peptide in the membrane, and residues considered to aid solubility may be added at that terminus to increase solubility. Exemplary ABC transporters that can be inhibited by these peptides include MDR1, MRP1, MRP2 and BCRP. The invention further provides nucleic acids encoding the peptides, expression cassettes comprising the nucleic acids, and host cells expressing the expression cassettes. The invention further provides a simple and inexpensive assay for determining whether a potential chemotherapeutic agent can inhibit the activity of P-gly-coprotein.