ATP-binding cassette (ABC) transporters are efflux transporters found in all organisms. These proteins are responsible for pumping xenobiotic and endogenous metabolites through extra- and intracellular membranes, thereby reducing cellular concentrations of toxic compounds. ABC transporters have been associated with drug resistance in several nematodes and parasitic arthropods. Here, the ability of ABC transporter inhibitors to enhance ivermectin (IVM) sensitivity was tested in larvae and adult females of Rhipicephalus (Boophilus) microplus. Larvae of susceptible and IVM-resistant tick populations were pre-exposed to sub-lethal doses of the ABC transporter inhibitors Cyclosporin A (CsA) and MK571, and subsequently treated with IVM in a Larval Packet Test (LPT). ABC transporter inhibition by both drugs significantly reduced the concentration for 50% lethality (LC(50)) values of four IVM-resistant populations but IVM sensitivity of a susceptible population remained unchanged. IVM sensitivity in adults was assessed through an artificial feeding assay. The addition of CsA to a blood meal substantially affected IVM toxicity in adult female ticks from a resistant population by reducing oviposition and egg viability, although it did not alter IVM toxicity in susceptible females. Three partial nucleotide sequences with similarity to ABC transporters were retrieved from the DFCI Boophilus microplus Gene Index (http://compbio.dfci.harvard.edu/index.html). Their transcriptional levels in the midgut of resistant and susceptible females were determined by quantitative PCR, showing that one of these sequences was significantly up-regulated in IVM-resistant females and suggesting its participation in IVM detoxification. We believe this work reports the first known evidence for the participation of ABC transporters in IVM resistance in R. microplus.