The role of antigen nonspecific cytokines in T- and B-lymphocyte responses is now well established. Interleukin-1 (IL-1) has been shown to augment B-cell maturation and proliferation. While the major source of IL-1 is from monocytes or macrophages, other cell types have been shown to produce IL-1-like cytokines. Epidermal cells produce a cytokine termed 'epidermal cell-derived thymocyte-activating factor' (ETAF) which is similar if not identical with monocyte-derived IL-1. In this report we show that ETAF induces polyclonal stimulation of murine B cells. We show that ETAF augments B cell proliferation and differentiation in the absence of any added antigens or mitogens. This activity can be partially inhibited by anti-IL-1 antibodies. ETAF appears to activate B cells directly, although its activity is increased in the presence of T cells. Thus, ETAF may be involved in local polyclonal antibody responses occurring in the skin.