The presence of P-glycoprotein (P-gp) and multiple drug resistance-associated protein (MRP) was examined in four human pancreatic adenocarcinoma cell lines (PANC-1, BxPC-3, AsPC-1, and Capan-1). Cellular accumulation of rhodamine 123 and [3H]vincristine were used to determine functional activity of P-gp and MRP, respectively. None of the cells showed any evidence of P-gp in the rhodamine 123 cellular accumulation assays. In contrast, PANC-1, BxPC-3 and AsPC-1 did display an increased accumulation of [3H]vincristine following treatment with either cyclosporin A or verapamil. Western blot analysis confirmed the expression of MRP, and little, if any, measurable P-gp in the cell lysates. These studies suggest that intrinsic drug resistance in pancreatic duct cancer may be due in part to the presence of MRP.