The invention provides monoclonal antibodies that specifically bind to CD122, which is one component of receptors for IL-2 and IL-15. The monoclonal antibodies have the capacity for substantial inhibition of both IL-2 and IL-15 mediated functions by inhibiting binding of these cytokines to their receptors. The monoclonal antibodies can be used for inhibiting undesired immune responses or treatment of cancer, among other applications.