IL-4 specifically induced IgE production by peripheral blood lymphocytes or by tonsil or spleen cells from healthy donors. IL-4-induced IgE synthesis was dependent on CD4+ T cells and monocytes and was blocked by IFN-y, IFN-a, and prostaglandin E-2 (PGE-2). These substances also inhibited IL4-induced CD23 expression and subsequent release of soluble CD23 (s-CD23). In addition, IgE production was blocked by F(ab')2 fragments of an mAb against CD23. In contrast, IL-5 enhanced IL4-induced IgE production, provided I L 4 was added at nonsaturating concentrations. This increase in IgE production correlated quantitatively with an enhanced release of s-CD23. Collectively, these results indicate that there is a correlation between s-CD23 release and IgE production. However, s-CD23 fractionated from supernatants of the lymphoblastoid cell line RPMI-8866 was ineffective in inducing IgE production in the absence of IL4, but acted synergistically with suboptimal concentrations of IL-4. In addition, it is demonstrated that alloreactive T-cell clones produced varying concentrations of I L 4 , IL-2, or IFN-y upon stimulation. Only supernatants of 2/4 of these T-cell clones induced a low degree of IgE synthesis, but in the presence of anti-IFN-y antibodies, all four supernatants induced a strong induction of IgE production. This IgE synthesis was blocked specifically by anti-IL-4 antibodies, indicating that IL-4 is the sole inducer of IgE synthesis. Our findings demonstrate that IL-4-induced IgE production involves complex interactions of T cells, B cells, and monocytes and is positively modulated by IL-5 and s-CD23 but down-regulated by IFN-y, IFNa, and PGE-2, respectively.