Interleukin-4 (IL-4) gene expression by stimulation with concanavalin A (Con A) in murine spleen cells was examined. The amount of IL-4 mRNA induced by Con A was greatest in spleen cells obtained from IgE high responder strains of mice. A trace amount of IL-4 mRNA was induced in spleen cells from IgE low responder (SJL) mice. The amount of IL-4 mRNA induced in spleen cells from an IgE intermediate responder (C57BL/6) was smaller than that from high responders, but significantly greater than that from low responders. Spleen cells from IgE nonresponders (SJA/9) developed only a negligible amount of IL-4 mRNA. IL-4 mRNA was detected in human peripheral blood mononuclear cells (PBMC) stimulated with Con A. The IL-4 gene expression seemed to be greater in PBMC obtained from highly atopic patients and was decreased in PBMC from individuals showing hypo-IgE immunoglobulinemia (less than 10 IU/ml). The results obtained in the present study may indicate that high and low IgE responder traits are determined depending on their levels of IL-4 mRNA expression.