A novel pathway of IgE-B cell differentiation has been identified. Engagement of the B cell antigen CD40 by F(ab')2 fragments of monoclonal antibody (mAb) 626.1 in the presence of recombinant interleukin 4 (rIL-4) induced intense IgE synthesis, but modest IgG synthesis, by highly purified human B cells. Surface IgE- B cells isolated by cell sorting were induced to produce IgE by mAb 626.1 and IL-4. Thus, IgE synthesis is unlikely to result from expansion of a B cell population precommitted to IgE in vivo. A neutralizing anti-IL-6 antibody strongly, but not completely, inhibited the IgE response. This indicates that autocrine production of IL-6 plays an important amplification role in IgE synthesis triggered by anti-CD40 mAb and IL-4. Although the exact role played by CD40 in IgE responses in vivo remains to be established, this T cell-independent system represents a useful model to characterize the biochemical and molecular events leading to IgE synthesis in human B cells.