Cellular homeostasis requires a balance between cell production, cell survival, and cell death. Production of natural killer (NK) cells from bone marrow precursor cells requires interleukin 15 (IL-15); however, very little is known about the factors controlling survival of mature NK cells in vivo. Because mice deficient in IL-15 (IL-15^−/− mice) fail to develop NK cells, it is not known whether mature NK cells can survive in an environment lacking IL-15. We hypothesized that IL-15 might indeed be required for survival of mature NK cells in vivo. Freshly isolated NK cells labeled with 5-(and-6)-carboxyfluorescein diacetate, succinimidyl ester (CFSE) were adoptively transferred into IL-15^−/− mice and littermate control (IL-15^+/−) mice. Within 36 hours after transfer, NK cells were detected in both IL-15^−/− and IL-15^+/− mice; however, significantly more (P < .003) CFSE-positive (CFSE⁺) NK cells were found in control mice than in IL-15^−/− mice. By 5 days, similar numbers of CFSE⁺ NK cells were still easily detected in IL-15^+/− mice, whereas no CFSE⁺ NK cells survived in IL-15^−/− mice. Furthermore, mice with severe combined immunodeficiency treated with the Fab fragment of a blocking antibody recognizing a signaling subunit of the IL-15 receptor, IL-2/15Rβ, had a significant (∼90%) loss of NK cells compared with control mice. Finally, NK cells from Bcl-2 transgenic mice that were adoptively transferred into IL-15^−/− mice did survive. These results show conclusively that IL-15 is required for mature NK cell survival in vivo and suggest that IL-15 mediates its effect on NK cell survival by means of Bcl-2.