IL-15 is critical for generation of multiple lymphoid subsets. Recent data have demonstrated a unique aspect of responses to IL-15, in that cells bearing the IL-15Rα chain can bind soluble IL-15 and "transpresent" the cytokine to other cells, allowing the latter to respond to IL-15. However, it is unclear whether IL-15 is normally secreted and then becomes bound to surface IL-15Rα on bystander cells, or whether transpresentation is mediated by the same cells which synthesize IL-15. Using mixed bone marrow chimeric mice, we present evidence for the latter model, showing that development of NK cells and memory phenotype CD8 T cells necessitates that both IL-15 and IL-15Rα be expressed by the same population of cells. These data argue that soluble forms of IL-15 are irrelevant for physiological responses to this cytokine, and the implications of this finding are discussed.