The present invention relates to the identification of an epitope in human Interleukin-15 (IL-15) that is responsible for binding to the interleukin-15 receptor .alpha.-chain. Two IL-15 regions are involved in the formation of this epitope: the first region (.sub.44LLELQVISL.sub.52, peptide 1) corresponds to a sequence located in the B helix and the second (.sub.64ENLIL.sub.68, peptide 2 or .sub.64ENLIIL.sub.69, peptide 2a) to a sequence located in helix C. Muteins displaying agonist or antagonist properties are described, and may be useful as therapeutic agents.