Abstract: Compounds disclosed which inhibit ABCB1 transporter protein are useful for treating diseases in which ABCB1 transporter protein mediates the disease state, including numerous cancers, including hematopoietic cancers, including various leukemias, especially T-lineage acute lymphoblastic leukemia, as well as cancerous tumors, especially forms which exhibit multiple drug resistance. Pharmaceutical compositions which comprise an inhibitor of ABCB1 transporter protein and at least one additional anticancer agent, optionally in combination with a pharmaceutically acceptable carrier, additive or excipient are another aspect of the present invention. A flow cytometry based, high-throughput screening (HST) assay that quantifies ABCB1 efflux is also disclosed. Methods of identifying inhibitors of ABCB1, ABCG2 and ABCC1 transporter proteins are also disclosed.