In the present study culture conditions resulting in optimal IgE synthesis by mononuclear cells (MNC) isolated from peripheral blood, tonsils or spleens from healthy nonallergic donors were investigated. The highest rate of IgE synthesis was obtained in a two-step culture system in which the MNC were preincubated with interleukin 4 (IL4; 200 U/ml) for 48 h, washed and subsequently incubated with IL4 (200 U/ml) for 9 days. Despite these culture conditions, IL4-induced IgE synthesis varied considerably (1-150 ng/ml) and MNC from 16/70 donors failed to produce IgE. Kinetic studies indicated that IL4 was required at the onset of the incubation phase. IgE synthesis was reduced by greater than 95% when addition of IL4 in the incubation period was delayed 24 h or more. IL4-induced IgE synthesis was blocked by interferon-gamma (IFN-gamma). This inhibition is most effective when IFN-gamma was added in the 48-h preincubation step or during the first 48 h of the incubation period. Interestingly, IL4 was found to block spontaneous and lectin- or factor-induced IFN-gamma production by MNC, purified CD3+, CD4+ or CD8+ Tcells. This down-regulatory effect of IL4 on IFN-gamma production occurred at the mRNA transcription level. Furthermore, it is shown that IL4 induced the release of soluble CD23 and that recombinant soluble CD23 enhanced IL4-induced IgE synthesis, but only when IL4 was present at suboptimal concentrations. Collectively, our data indicate that IL4 and IFN-gamma regulate the level of IgE synthesis by influencing each other's activities reciprocally during the first 3 days of the culture.