Although the receptor for interleukin-4 (IL-4R) is highly expressed on solid human cancer cells, its significance and internalization function is still unclear. To address these issues, we reconstituted Chinese hamster ovarian (CHO-K1) cells with various components of the IL-4R by transient transfection and performed internalization assays using radiolabeled IL-4. Radiolabeled IL-4 internalized through the IL-4Ralpha chain in a time-dependent manner. When the IL-4Ralpha chain was cotransfected with the IL-13Ralpha1 or -gamma(c) chain, the IL-4 internalization level was identical to IL-4Ralpha transfectants, suggesting that the IL-4Ralpha chain plays a major role in IL-4 internalization. These results were confirmed by determining the cytotoxicity of a chimeric protein composed of IL-4 and a mutated form of Pseudomonas exotoxin [IL4(38-37)-PE38KDEL] in CHO-K1 cells transfected with increasing concentrations of IL-4Ralpha cDNA. To use the internalization property of the IL-4Ralpha chain in the context of IL-4R-targeted cytotoxin therapy, we transiently transfected pancreatic and brain tumor cells with IL-4Ralpha chain. Surprisingly, these tumor cells acquired 4-75-fold higher binding activity to IL-4 compared with control cells. Consequently, the cytotoxic activity of IL-4 toxin to these cancer cells was enhanced 5-13-fold compared with control cells as assessed by protein synthesis inhibition and clonogenic assays. Taken together, a combination approach that involves transfer of the IL-4Ralpha gene and IL-4R-targeted cytotoxin therapy may serve as a novel approach for cancer therapy.