We studied the expression of the receptor of interleukin (IL-4), one of the T cell growth factors, on fresh peripheral blood leukemic cells from adult T-cell leukemia (ATL) patients. Flow cytofluorometric analysis with a monoclonal antibody to the IL-4 receptor (IL-4R) were used to investigate whether expression of IL-4R on ATL cells is different from that on normal lymphocytes and other types of leukemic cells. Leukemic cells from acute type ATL patients synthesize IL-4R without stimulation, at levels much higher than normal resting lymphocytes and other types of leukemic cells. Furthermore, leukemic cells from acute type ATL showed higher IL-4R expression than that of chronic type ATL or human T-cell leukemia virus type I carriers. In addition, there was correlation between expression of IL-4R on the cell surface and the proliferative response to IL-4. Both IL-4 and IL-2 induced upregulation of IL-4R on activated normal T cells but not on ATL cells. These results suggest that abnormal expression of IL-4R may display different biological activities in ATL compared with other types of leukemia. Furthermore, the high expression of IL-4R in ATL may be involved in the proliferation of leukemic cells and the leukemogenesis in this disease.