Interleukin-13 (IL-13) is a recently described lymphokine. Like IL-4, IL-13 induces the production of IL-1 receptor antagonists and suppresses the production of inflammatory cytokines by macrophages and monocytes. In this study, we investigated the effects of IL-13 on the migration and proliferation of human umbilical vein endothelial cells (HUVECs) and human dermal microvascular endothelial cells (HMVECs). We examined the effect of IL-13 on endothelial chemotaxis under two conditions: first, endothelial cells were exposed to a combination of IL-13 and a known chemotaxin, basic fibroblast growth factor (bFGF); second, endothelial cells were exposed to IL-13 alone. The effects of IL-13 on endothelial proliferation were also examined. IL-13 showed no inhibitory effects on bFGF-induced chemotactic activity on either HUVECs or HMVECs. IL-13 demonstrated chemotactic activity for HUVECs and HMVECs. For both cell types, peak chemotactic activity was at or above that of bFGF (60 nM). By varying concentrations of IL-13 in the upper and lower wells of the chemotaxis chambers, we found IL-13 to be chemotactic, and not chemokinetic, for HUVECs and HMVECs. IL-13 did not induce mitogenesis of either HUVECs or HMVECs. Although IL-13 has been shown to have many anti-inflammatory actions, these results suggest a novel role for IL-13 as a proinflammatory proangiogenic cytokine.