The transport of the antiparasitic agents, ivermectin, selamectin and moxidectin was studied in human intestinal epithelial cell monolayers (Caco-2) and canine peripheral blood lymphocytes (PBL). Both models expressed the mdr1-coded 170 kDa ATP-binding cassette (ABC) transporter P-glycoprotein (P-gp). Fluxes of the P-gp substrate rhodamine-123 (Rh-123) across Caco-2 monolayers showed that ivermectin and selamectin acted as potent P-gp inhibitors with IC50 values of 0.1 microm. In contrast, moxidectin was a weaker P-gp inhibitor with an IC50 of 10 microm. The transport of radiolabelled ivermectin, selamectin and moxidectin through Caco-2 monolayers showed that ivermectin, selamectin and moxidectin were P-gp substrates with secretory/absorptive ratios of 7.5, 4.7 and 2.6 respectively. Secretory transport of [3H]-ivermectin and [3H]-selamectin was blocked by the P-gp inhibitor, verapamil. Ivermectin and selamectin inhibited the efflux of Rh-123 from PBL and the concentration of inhibition was similar to that of verapamil. In contrast, moxidectin did not have a significant effect on Rh-123 efflux from PBL. The data suggest that ivermectin and selamectin are potent P-gp substrates, while moxidectin is a weak one.