Membrane-bound and soluble interleukin-15 (IL-15)/IL-15 receptor alpha (Ralpha) complexes trigger differential transcription factor activation and functions on human hematopoietic progenitors. Indeed, human spleen myofibroblasts (SMFs) are characterized by a novel mechanism of IL-15 trans-presentation (SMFmb [membrane-bound]-IL-15), based on the association of an endogenous IL-15/IL-15Ralpha complex with the IL-15Rbetagamma c chains. SMFmb-IL-15 (1) induces lineage-specific signaling pathways that differ from those controlled by soluble IL-15 in unprimed and committed normal progenitors; (2) triggers survival and proliferation of leukemic progenitors expressing low-affinity IL-15R (M07Sb cells); (3) causes only an antiapoptotic effect on leukemic cells expressing high-affinity receptors (TF1beta cells). This behavior is likely due to the IL-15Ralpha chain present on these cells that interact with the SMFmb-IL-15, inhibiting signal transducer and transcriptional activator 5 (STAT5) activation. On the other hand, the soluble IL-15/IL-15Ralpha complex (hyper IL-15) displays a dominant pattern of action, activating only those cells expressing low-affinity IL-15R (IL-15Rbetagamma c). Thus, hyper IL-15 induces antiapoptotic effects on M075b cells and the up-regulation of STAT6 activation on adult peripheral blood (PB) pre-natural killer (NK) committed progenitors. The latter effect using 100-fold concentrations of recombinant (r)-IL-15. In conclusion, SMFmb-IL-15 and soluble IL-15Ralpha/IL-15 complexes seem to play a pivotal role in the control of the survival, proliferation and differentiation of both normal and leukemic circulating progenitors, highlighting new functions of IL-15 and of IL-15Ralpha.