Fluoroquinolones such as ciprofloxacin are secreted into the intestinal lumen and this is an important route for whole body drug elimination. Our own work has shown that ciprofloxacin is not secreted by MDR1-transfected MDCKII epithelial monolayers nor is it subject to inhibition by MRP inhibitors such as MK-571 (Lowes and Simmons, 2002). Recently intestinal secretion via BCRP has been suggested as the major route for intestinal secretion (Merino et al, 2006). The purpose of the present study was therefore to evaluate the role of BCRP in secretion and to investigate whether secretion in the human intestinal cell model, Caco-2, is mediated solely by BCRP. Epithelial monolayers of human Caco-2 cells native MDCKII cells , murine bcrp-MDCKII cells and human BCRP-MDCKII cells were grown on Transwell polycarbonate filters after high-density seeding (Lowes and Simmons, 2002) and transepithelial fluxes in the apical to basal (Ja-b) and basal to apical (Jb-a) surfaces determined using ¹⁴C-ciprofloxacin (total concentration 10μM). Whereas in native MDCK layers no net secretion of ciprofloxacin was evident (Ja-b = 0.174 ± 0.019 nmol/cm²/hr, Jb-a = 0.127 ± 0.014 nmol/cm²//hr, N = 3 experiments of 3 replicates, means ± S.E.M.), a significant net secretion was observed in mbcrp-MDCK II epithelia (Jnet= Jb-a – Ja-b, 0.187 ± 0.02 nmol/cm²//hr N = 3, p<0.01, single tailed T test vs 0). In contrast, hBCRP-MDCKII epithelial layers showed no net secretion (Jnet= -0.07 ± 0.06 nmol/cm²//hr, N = 3). Ciprofloxacin secretion by mbcrp-MDCKII cell-layers was inhibited by a specific BCRP inhibitor (1μM Ko143, Allen et al 2002) from control values of Jnet of 0.187 ± 0.01nmol/cm²//hr, n= 3 to 0.013 ± 0.012 nmol/cm²//hr, n= 3, p<0.01, unpaired means T test). For 2 strains of Caco-2 cells , ciprofloxacin secretion was observed (Jnet= 0.39 ± 0.07 nmol/cm²//hr, n= 3 for both ) but secretion was only partially inhibited by 1 μM Ko143 by 10 % and 53% respectively. This data is therefore consistent with both BCRP mediated and BCRP-independent transport pathways. In order to test whether MRP4 may mediate ciprofloxacin secretion, cellular accumulation of ¹⁴C-ciprofloxacin (total concentration 10μM) by MRP4 transfected HEK cells was determined. Ciprofloxacin accumulation was reduced by MRP4 transfection consistent with ciprofloxacin being an MRP4 substrate. However, though MK571 was an effective MRP4 inhibitor in MRP4-HEK cells, it did not inhibit transepithelial ciprofloxacin secretion by Caco-2 cells. We conclude that ciprofloxacin is a substrate of for both BCRP and MRP4 but that the substantial fraction of ciprofloxacin secretion by Caco-2 epithelia is mediated by neither BCRP nor MRP4