Interleukin-15 is a new cytokine that stimulates the proliferation of T cells and other cells of the immune system. Some of the biological properties of interleukin-15 overlap that of interleukin-2. Using murine models, the present studies have shown that interleukin-15, in vivo, is three to four times more potent than interleukin-2 in generating cytolytic effector splenocytes that lyse YAC target cells. It is approximately one-third as potent as interleukin-2 in inducing specific cytolytic cells that lyse allogeneic target cells. Interleukin-15 is approximately half as potent as interleukin-2 in suppressing pulmonary metastasis induced by MCA-205 tumor cells. The dose of interleukin-15 required to induce pulmonary vascular leak in mice is six times higher than that required for interleukin-2. These results support the view that interleukin-15 exhibits a therapeutic index that is superior to interleukin-2.