Specific monoclonal antibodies (mAb) directed against the CD23 antigen were used to study human interleukin 4 (hIL4)-induced IgE production by blood and tonsillar mononuclear cells. Both peripheral blood and tonsillar mononuclear cells stimulated by hIL4 expressed membrane CD23 as detected by the binding of all anti-CD23 mAb. Nevertheless, two sets of anti-CD23 mAb could be distinguished. The first set, including mAb 25, was able to decrease significantly hIL4-induced IgE synthesis by mononuclear cells. The second set, including EBVCS#1, did not affect hIL4-induced IgE synthesis. All the anti-CD23 mAb were able to bind specifically to a human B cell line expressing recombinant CD23. Inhibition experiments revealed that the two sets of anti-CD23 mAb did not recognize the same epitope on the CD23 antigen. In fact, all the anti-CD23 mAb, except EBVCS#1, were able to inhibit IgE binding to CD23 on RPMI 8866 cells. Moreover, the first set of antibodies, which decreased IgE production, was able to up-regulate membrane CD23 expression on hIL4-stimulated tonsillar mononuclear cells. Conversely, EBVCS#1, which had no effect on IgE production, did not affect hIL4-induced CD23 expression. These results indicate that CD23 plays a key role in human IgE synthesis.