Antagonists of mammalian interleukin-15 ("IL-15") are disclosed and include muteins of IL-15 and modified IL-15 molecules that are each capable of binding to the IL-15R.alpha.-subunit and that are incapable of transducing a signal through either the .beta.- or .gamma.-subunits of the IL-15 receptor complex. Also included are monoclonal antibodies against IL-15 that prevent IL-15 from effecting signal transduction through either the .beta.- or .gamma.-subunits of the IL-15 receptor complex. Methods of treating various disease states are disclosed, including treating allograft rejection and graft-versus-host disease,