Multidrug-resistance–associated protein, MRP8/ABCC11 (ABCC11), is an efflux pump for nucleotide analogues and 5-fluoro-2′-deoxyuridine 5′-monophosphate (FdUMP). To test whether ABCC11 directly confers 5-fluorouracil (5-FU) resistance, we used the 5-FU–resistant subline PC-6/FU23-26 selected from PC-6 human small-cell lung cancer cells by 5-FU and found that it increases the resistance by ∼25-fold. The intracellular FdUMP accumulation was reduced in PC-6/FU23-26 cells concomitant with the overexpression of the ABCC11 gene. These findings suggest that ABCC11 confers 5-FU resistance in the sublines by enhancing the efflux for the active metabolite FdUMP. Previously, methotrexate also increased the efflux by ABCC11, and we found cross-resistance to methotrexate in PC-6/FU23-26 cells. To confirm our hypothesis, we examined whether decreasing the expression of ABCC11 in PC-6/FU23-26 cells by small interfering RNA altered the cytotoxicity to 5-FU and methotrexate and found that this enhanced 5-FU and methotrexate cytotoxicity in PC-6/FU23-26 cells. These data indicate that expression of the ABCC11 gene is induced by 5-FU, and that ABCC11 is directly involved in 5-FU resistance by the efflux transport of the active metabolite FdUMP. [Mol Cancer Ther 2007;6(1):122–7]