HAM/TSP affects about 1% of patients infected with the human T lymphotropic virus type 1 (HTLV-I), manifesting as progressive myelopathy. Several features of the immune response in HAM/TSP indicate that IL-15 may be critical to its pathogenesis. HTLV-I transactivates IL-15 and IL-15 R alpha expression through its tax protein, and supports the activation of lymphocytes in HAM/TSP, as evidenced by their spontaneous proliferation in ex vivo culture. We showed that the antibody HuMiK-Beta1 could inhibit this spontaneous lymphoproliferation. In addition, IL-15 has been shown to have a preferential positive effect on the survival of CD8+ T-cells, especially those of the memory phenotype. In HAM/TSP, an extraordinarily high frequency of tax specific CD8+ T-cells in the peripheral blood and cerebrospinal fluid is postulated to participate in the damage to the central nervous system. We have demonstrated that the addition of Hu MiK-Beta1 to ex vivo cells from HAM/TSP patients led to a marked decline in tax specific CD8+ T-cells.