IL-13 is a relatively novel cytokine produced by activated T cells. IL-13 inhibits the production of proinflammatory cytokines and chemokines by activated monocytes, induces B cell proliferation and differentiation, including IgE production, and the expression of certain adhesion molecules on endothelial cells. All these biological properties of IL-13 are shared with IL-4, but in contrast to IL-4, IL-13 does not act on T cells. In this review, the similarities in structure and biological function of IL-13 and IL-4 and the commonalities of their receptors are summarized. The potential role of IL-13 in allergic responses is briefly discussed.