Recently, interleukin-4 (IL-4) has been clarified as having T-cell growth factor activity; therefore, it becomes of interest whether IL-4, as well as interleukin-2 (IL-2), affects the proliferation of leukemic cells derived from mature T cells. In the present study, we describe a case of chronic T-lymphocytic leukemia (T-CLL) with monoclonal proliferation of human T-lymphotropic retrovirus (HTLV)-I or HTLV-II negative CD3(+)4(+)8(-) T cell expressing IL-2 receptors without stimulation. Radiolabeled IL-2 binding assay revealed 750 high-affinity and 6,750 low-affinity binding sites per cell. In accordance with the expression of high-affinity IL-2 receptors, the leukemic cells proliferated in response to exogenous IL-2 without prior stimulation. In addition, exogenous IL-4 also induced their proliferation. Moreover, IL-2 and IL-4 exerted a synergistic effect on the leukemic cell proliferation. Although the expression of IL-2 or IL-4 mRNA was not detected in fresh leukemic cells, the expression of IL-2 mRNA, but not IL-4 mRNA, was induced by phytohemagglutinin stimulation, and the leukemic cells proliferated. These findings suggest that not only IL-2, but also IL-4 are involved in the proliferation of leukemic cells of T-CLL.