The cytokine IL-4 has unique effects on human endothelial cells. It specifically increases expression of vascular cell adhesion molecule (VCAM)-1 promoting adhesion of lymphocytes but not neutrophils, and causes profound effects on the morphology of endothelial monolayers characterized by formation of cell clusters and the appearance of holes in the cultured monolayer. In this study we show that the effects of IL-13 on human umbilical vein endothelial cells (HUVEC) are indistinguishable from those of IL-4. Both cytokines induce the same morphological changes in cultured HUVEC monolayers which are distinct from any other cytokine. In addition, IL-13 and IL-4 stimulate comparable levels of VCAM-1 expression with similar time kinetics, but at doses 10-fold less than those required for B cell activation and proliferation. Using a combination of mutant IL-4 antagonists and mAb to the IL-4Ra chain (CD124), we show that expression of IL-4Rα is essential for HUVEC responses to both IL-4 and IL-13, consistent with this receptor subunit being a component of the receptors for both cytokines. In contrast, the common γ chain (γc), which is a component of the classical IL-4 receptor, was not detected on endothelial cells by flow cytometry or immunogold histochemistry. In addition, RT-PCR showed extremely low or absent γc mRNA, consistent with the absence of detectable surface protein. These results strongly suggest that the cytokines IL-4 and IL-13 are both important in modulating endothelial cell function, and may act through a single receptor complex on human endothelial cells that includes the IL-4Rα chain but not the γc chain.