Interleukin-2 (IL-2), which is a growth factor for T lymphocytes, can also sometimes be inhibitory. Thus, the proliferation of CD8⁺ T cells in vivo is increased after the injection of a monoclonal antibody that is specific for IL-2 (IL-2 mAb), perhaps reflecting the removal of IL-2–dependent CD4⁺ T regulatory cells (T regs). Instead, we show here that IL-2 mAb augments the proliferation of CD8⁺ cells in mice simply by increasing the biological activity of preexisting IL-2 through the formation of immune complexes. When coupled with recombinant IL-2, some IL-2/IL-2 mAb complexes cause massive (>100-fold) expansion of CD8⁺ cells in vivo, whereas others selectively stimulate CD4⁺ T regs. Thus, different cytokine-antibody complexes can be used to selectively boost or inhibit the immune response.