Hu-Mikβ1 inhibits IL-2– and IL-15–induced proliferation of 32Dβ cells that express human IL-2/IL-15Rβ and the murine common γ-c. The dose-response effects of addition of Hu-Mikβ1 Ab were evaluated on IL-2 (1.0 ng/mL)– and IL-15 (20 ng/mL)–induced proliferation of 32Dβ cells. In these cells, which express only human IL-2/IL-15Rβ and the common γ-c, but not IL-2Rα or IL-15Rα, IL-2– and IL-15–induced proliferations were inhibited by the addition of Hu-Mikβ1.