Interleukin 2 receptors (IL-2-R) are composed of (at least) two chains: the alpha chain (or p55 or Tac) and the beta chain (or p70 or p75). The association of both chains constitutes high affinity IL2-R complexes (Kd congruent to 10-100 pM), whereas the alpha-chain or the beta chain alone bind IL-2 with low (Kd congruent to 5-50 nM) and intermediate (Kd congruent to 1 nM) affinities, respectively. The beta chain (but not the alpha chain) mediates IL-2 internalization and signal transduction (proliferation or differentiation) to the cell, thus only high and intermediate affinity IL-2-R are functional. High affinity IL-2-R are expressed on activated normal and leukemic T and B lymphoid cells. Low affinity IL-2-R (Tac alone) are seen on stimulated T cells, B cells, and monocytes, as well as on various leukemic cells. The p70 chain alone is constitutively expressed by subsets of normal resting T and B cells, LGL and NK cells and we have recently demonstrated its presence on leukemic cells from various hemopoietic lineages. The finding of IL-2-R on non-lymphoid cells may disclose new functions for IL-2.