A humanized version of the immunoglobulin (Ig) G1 monoclonal antibody Mik-Beta-1 (Hu-Mik-Beta-1) directed against CD122, the beta-subunit shared by the interleukin-2 (IL-2) and IL-15 receptor (IL-2/IL-15Rbeta). Upon intravenous infusion, Hu-Mik-Beta-1 binds to CD122 expressed on certain tumor cells. This blocks the binding of the inflammatory cytokines IL-2 and IL-15 to IL-2R and IL-15R, respectively, and prevents IL-2/IL-2R- and IL-15/IL-15R-mediated signaling. This may inhibit the proliferation of CD122-expressing tumor cells. In addition, blocking CD122 on T-lymphocytes prevents the over-activation of T-lymphocytes in various T-cell mediated autoimmune diseases, which is predominantly mediated by IL-15/IL-15R signaling. CD122, involved in both IL-2 and IL-15 signaling, is overexpressed on certain cancer cells, such as those found in T-cell large granular lymphocyte (T-LGL) leukemia.