BSF-l/lnterleukln-4, a product of activated T cells, has multiple biological activities that affect cells of most hematopoletic lineages. Among these Is the ability of BSM to costimulate the growth of mast cells and regulate the production of IgE. We demonstrate here that BSF-1 mRNA is expressed by a majority of transformed mast cell lines and by 5 IL-3dependent nontransformed mast cell Ilnes. BSF-1 activity, Including the ability to enhance the growth of IL-8dependent mast cells, was detected In the supernatants of transformed mast cells. The role of BSM as a mast cell growth factor, Its constltutlve production by transformed mast cells, and the lack of IL-3 production by most of these cells raise the possibility that BSF-1 may act as an autocrlne growth factor for some transformed mast cells. Furthermore, production of BSM mRNA by nontransformed cells indicates that mast cells may be an important physiologic source of this factor.