Clonal expansion of T lymphocytes of the helper/inducer class is generally thought to be mediated by an interleukin 2 (IL-2)-dependent autocrine mechanism. Thus, T cells stimulated by antigens or mitogenic lectins secrete IL-2 and, under appropriate conditions, express membrane receptors for IL-2, and the specific hormone-receptor interaction induces cellular proliferation. Recent studies indicate that B-cell stimulatory factor 1 (BSF-1) is secreted by T cells and is capable of stimulating T-cell proliferation. We now report that BSF-1 and not IL-2 is the sole autocrine growth factor for certain cloned lines of inducer T lymphocytes. On stimulation by the lectin concanavalin A, anti-receptor antibody, or specific antigen with antigen-presenting cells, such clones secrete a lymphokine that stimulates DNA synthesis by the "IL-2 indicator line," HT2, but is identified as BSF-1 by specific inhibition with monoclonal antibodies. The proliferative response of such BSF-1-secreting clones to receptor-mediated signals is dependent on BSF-1 and not IL-2. These results demonstrate a function of BSF-1 and confirm the existence of a previously unknown autocrine pathway of T-cell activation.