Leukemic cells in the peripheral blood of a patient with adult T cell leukemia (ATL), which expressed the Tac antigen/interleukin 2 (IL2) receptor, were investigated in vitro for autocrine growth by IL 2. The cells showed spontaneous proliferation in mitogen-free medium. The spontaneous proliferation of the cells was inhibited by monoclonal anti-IL 2 or anti-Tac antibody. These cells were found to produce messenger RNA for IL 2 and secrete IL 2 during short-term culture in the same medium. Recombinant IL 2 and IL 2 secreted by the cells enhanced the proliferation of the cells in a dose-dependent manner when added to the initial culture. These findings demonstrate that an autocrine mechanism by IL 2 is involved in the proliferation of ATL cells during short-term culture.