In a previous study, we reported that adult T-cell leukemia (ATL) cells produce interleukin 1 (IL1)-like factors that stimulate murine thymocyte proliferation, the production of interleukin 2 (IL2), and the expression of IL2 receptors (IL2R) on normal human T-cells in the presence of concanavalin A. In this communication, we studied the effect of IL1 on the growth of ATL cells in vitro. When ATL cells freshly obtained from patients were cultured with recombinant (r) human IL1 alpha, IL1 beta, or IL1-like factors produced by ATL cell lines, the growth of ATL cells was stimulated in a concentration-dependent manner. Maximum stimulation was observed at a concentration of 50-100 units/ml of IL1. The expression of IL2R on ATL cells was also enhanced by IL1, but the production of IL2 was not induced. These effects of rIL1 alpha or beta were specifically inhibited by anti-IL1 alpha or anti-IL1 beta antibody. Furthermore, the spontaneous growth of ATL cells was also inhibited by anti-IL1 alpha antibody, but not by anti-IL1 beta antibody. ATL cells exhibited enhanced expression of IL1 receptors on their surface as detected by the binding of 125I-labeled rIL1 alpha. These results suggest that IL1 alpha produced by ATL cells stimulates the growth of ATL cells by an autocrine mechanism.