We report intriguing aspects of the contribution of IL-15Ralpha to IL-15 functions. Consistent with high-affinity interactions between IL-15 and IL-15Ralpha, these two molecules form stable complexes on the cell surface of activated monocytes. The formation of IL-15/IL-15Ralpha complexes on cell surfaces induces a trans-endosomal recycling of IL-15 leading to the persistence of surface-bound IL-15 due to the constant reappearance of IL-15 on plasma membranes. This complex contributes to the long survival of T cells expressing IL-15Ralpha after IL-15 withdrawal. Finally, these complexes on activated monocytes present IL-15 in trans to target cells such as CD8(+) T cells that express only IL-2/15Rbeta and gammac upon cell-cell interaction.