In order to elucidate the factors responsible for altered immunoglobulin production in patients with IgA nephropathy (IgAN), the in vitro effects of IL-4 and interferon-gamma (IFN-gamma) on the synthesis of IgE and IgA by peripheral blood mononuclear cells (PBMC) were studied. Spontaneous IgE and IgA synthesis by PBMC was significantly increased in patients with IgA nephropathy compared with controls. The maximum amounts of IgA and IgE synthesis by PBMC after stimulation with IL-4 were almost the same both in patients with IgAN and in controls. The enhancement rate of IL-4-induced IgE and IgA synthesis was significantly lower in IgAN than in the controls, suggesting in vivo preactivation of PBMC in IgAN patients. IFN-gamma suppressed IgA and IgE synthesis by PBMC from IgAN patients as well as controls. However, the suppressive effect on IgE synthesis was less prominent in patients with IgAN. These results suggested that altered IL-4 action might be involved in the development of IgA nephropathy.