Several of the ATP-binding cassette (ABC) transporters confer resistance to anticancer agents and/or antiviral agents when overexpressed in drug-sensitive cells. Recently a MRP1 (ABCC1) tricyclic isoxazole inhibitor, LY475776 was shown to be a glutathione-dependent photoaffinity label of human MRP1 and showed poor labeling of murine mrp1, an ortholog that does not confer anthracycline resistance. In the present study, the specificity of LY475776 was examined for its ability to modulate or photolabel orthologs of MRP1 and several other drug efflux transporters of the ABC transporter family. LY475776 modulated MRP1 and Pgp-mediated resistance (MDR, ABCB1) in, respectively, HeLa-T5 and CEM/VLB(100) cells to both vincristine and doxorubicin. LY475776 photolabeled 170kDa Pgp and was inhibited by the potent Pgp inhibitor LY335979 (Zosuquidar.3HCl). The labeling of the 190kDa MRP1 protein in membranes of HeLa-T5 cells was inhibited by substrates of MRP1 such as leukotriene C(4), vincrisine, and doxorubicin and by the inhibitor, MK571. LY475776 did not photolabel human MRP2 (ABCC2), MRP3 (ABCC3), MRP5 (ABCC5) or breast cancer resistance protein (ABCG2). Because LY475776 photolabels murine mrp1 less well than human MRP1 and binds to a region believed important for anthracycline binding, studies were conducted with monkey and canine MRP1 which also show a reduced ability to confer resistance to anthracyclines. Unlike murine mrp1, both orthologs were photolabeled well by LY475776. These studies indicate that the specificity of LY475776 is fairly limited to Pgp and MRP1 and further studies will help to define the binding regions.