Hapten-specific CD4+ T helper (Th) lines generated by repeated stimulation with hapten-modified, cultured Langerhans' cells (cLC) release interleukin (IL 4) (B cell stimulatory factor 1) but not detectable IL 2 into the culture media. The growth of Th cells in response to hapten-modified cLC was inhibited by an anti-IL 4 monoclonal antibody (mAb) but not by mAb to either IL 2 or the p55 chain of the IL 2 receptor. Furthermore, these cells could be stimulated to proliferate by concanavalin A and IL 1. These results indicate that IL 4 is the autocrine growth factor for these Th lines and that IL 1 plays a critical role in their growth. The Th cells exhibited 1,500-10,000 high-affinity IL 4 receptors cell. When cultured with syngeneic, hapten-modified, small resting B cells, Th cells caused specific IgE production of up to 20 ng/10(4) B cells. Thus, IL 4 producing Th lines appear to result from their selective stimulation by cLC, suggesting that T cell responses elicited in this way profoundly influenced the B cell isotype pattern.