Twelve patients with T cell large granular lymphocyte leukemia and associated hematocytopenia were treated in a phase I dose-escalation trial with the murine monoclonal antibody Mikβ1. Mikβ1 identifies CD122, the β-subunit shared by the IL-2 and IL-15 receptors. At the doses administered in this study the antibody inhibited the actions of IL-15 on both natural killer and T cells and that of IL-2 when the intermediate-affinity IL-2 receptor was expressed. Mikβ1 treatment was not associated with significant toxicity or with the development of an immune response to the infused monoclonal antibody. At these doses of Mikβ1, >95% saturation of the IL-2/IL-15β receptor (CD122) on the surfaces of the leukemic cells was achieved. Furthermore, in seven patients this led to the down-modulation of the receptor from the surfaces of the leukemic cells. Nevertheless, no patients manifested a reduction in peripheral leukemic cell count or an amelioration of their hematocytopenia. This latter observation may reflect the fact that the monoclonal T cell large granular lymphocyte leukemia leukemic cells of the patients did not produce IL-2 or IL-15 or require their actions for cell survival. In light of the lack of toxicity and lack of immunogenicity of the antibody observed in the present study and the role for IL-15 in the pathogenesis of autoimmune diseases, clinical trials should be performed using the humanized version of Mikβ1 in groups of patients with human T cell lymphotropic virus I-associated myelopathy/tropical spastic paraparesis, rheumatoid arthritis, multiple sclerosis and refractory celiac disease.