IL-15, a cytokine with biological functions on cells of lymphoid lineage similar to those of IL-2, mediates its activities through the β and γ chains of the IL-2/15R and its own α chain. Unlike IL-2, IL-15 also binds to endothelial cells with high affinity. We report here that IL-15 is a stimulator of angiogenesis in vivo. When injected subcutaneously into nude mice, IL-15 consistently induced neovascularization of Matrigel plugs. Endothelial cells were found to express the IL-15R α chain and the IL-2/15R β and common γ chains. IL-15 induced the rapid tyrosine phosphorylation of proteins in endothelial cells, but did not stimulate endothelial cell proliferation in vitro. These findings document a previously unrecognized biological property of IL-15 and emphasize the role of IL-15 as an important mediator outside the immune system.